Microleakage at the cervical margin of Class II composite restorations with different intermediate layer treatments

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I. INTRODUCTION

Tooth-colored posterior restorations are now chosen as the first choice of caries treatment by many patients. Most popular tooth-colored restoration is the direct composite. The current composites have been improved, demonstrating proper wear resistance for the posterior regions. When all margins of composite restorations are located in enamel, satisfactory esthetics and longevity have been reported in small restorations1-4). But in larger restorations where cervical margins are located in dentin, long-term success is more difficult to attain5-7). Despite significant improvements in adhesive dentin bonding, polymerization shrinkage continues to rupture the resin-tooth bond, producing a gap. The ensuing microleakage into this gap results in postoperative sensitivity and increased risk of secondary caries8-10).

Techniques proposed to eliminate this leakage have included incremental placement, clear matrices and reflecting wedges, beta quartz inserts, the use of autopolymerizing composite, pulsed/stepped curing, and sandwich techniques incorporating other materials as an intermediate layer11).

Intermediate layer of restorative material has been suggested as a means to improve both marginal integrity and adaptation of the high viscosity composite to cavity walls. Restorative materials advocated for this purpose have included flowable composites, compomers, autopolymerizing composites, and conventional and resin-modified glass ionomer cements12).

Flowable composites have reduced physical properties when compared to standard restorative composites, suffer from greater polymerization shrinkage, possess an increased coefficient of thermal expansion, and are not recommended for use in high stress applied locations. But used as an intermediate layer, their proposed mechanism of reducing microleakage is related to their increased elasticity, which is thought to compensate for the polymerization contraction stresses of the final restorative composite13). Additionally, flowability of this material is able to fill the corner of cavities more efficiently.

Glass ionomers including resin-modified glass ionomers have many excellent physical properties which are long-term molecular bonding to dentin and enamel, bacteriostatic action, thermal expansion similar to that of enamel and dentin, and a slow setting reaction with low setting shrinkage14).

Compomers are purposed to offer less stress at the resin-dentin interface because of their slower initial setting reaction. Additionally postplacement expansion of compomers from water sorption is believed to offset their initial shrinkage12).

If using any of these intermediate layers prior to placement of a composite, they could be helpful to eliminate microleakage.

The concept of using glass ionomers to bond composite resins to dentin was first introduced by McLean for Class III and Class V cavities15). Some authors16-18) tested this group of materials in an open-sandwich technique to seal the cervical cavity margins in Class II restorations. This concept failed clinically when conventional glass ionomers were used, mainly because of continuous loss of materials16). But recently introduced resin-modified glass ionomers and compomers show improved mechanical and physical properties and better control during clinical handling17). Some studies have indicated that these materials may improve the marginal adaptation of Class II restorations when a open-sandwich technique is used18,19). Whether resin-modified glass ionomers or compomers should be used for such restorations is difficult to decide, for different factors have to considered when selecting a materials for sandwich restorations.

The purpose of this study was to determine whether intermediate layers of flowable composite, compomer and resin modified glass ionomer prior to placement of condensable composites would eliminate or significantly decrease microleakage at the gingival margin of Class II direct posterior composite restorations placed on dentin.

II. MATERIALS AND METHODS

III. RESULTS

Figure 1 is photographs of specimens representing the microleakge scores. Results of dye leakage test and mean rank of Kruskal-Wallis test are shown in table 1.

Fig. 1

Scores of microleakage (×25): (a) score 1, (b) score 2, (c) score 3

Table 1

Distribution of microleakage scores and mean rank at gingival margins

Any sample didn't show perfect marginal seal. 80% samples from Group I leaked to the axial wall and in most cases, leakage extended to dentinal tubules. Similary, Group III leaked to the axial wall in majority of samples (70%). In Group II, 60% samples leaked within gingival wall and in Group IV, only one sample leaked to axial wall.

Figure 2 graphically presents the mean scores in all groups. Group IV which was treated with RMGIC as intermediate layer demonstrated significantly less microleakage (p<0.05). The results by the Mann-Whitney test demonstrated statistically significant differences when the comparisons were made between the margins with compomer as intermediate layer and resin-modified glass ionomer, the former demonstrated more leakage than the latter two groups (p<0.05) (Table 2).

Fig. 2

Mean scores of microleakage of each groups ar gingival margins.

Table 2

Statistical analysis of microleakage at gingival margin between each group by Mann-Whitney test(p<0.05)

VI. DISCUSSION

The marginal sealing ability of three different intermediate layer which were applied prior to placement of condensable composites on cervical margins located below CEJ of class II composite restorations, were evaluated in this study. In spite of many advantages, direct composite restorations have been limitedly used because of polymerization shrinkage. The polymerization shrinkage of composites has been reduced but not yet suppressed in commercially available products20). Consequently, stresses induced by polymerization and their potentially damaging effect on restoration adaptation still restrain a simple and safe application of direct composite restoration technique in large and deep Class II cavities21). In many earlier studies16-18), Class II composite restorations where cervical margins are located on enamel revealed good marginal sealing ability. But cervical margins were located below CEJ, none of the restorative techniques demonstrated a good sealing ability with moderate to severe microleakage.

The finding of this study that all experimental groups and control group showed extensive amounts of dye leakages in cervical margins confirms the results of many other studies22-25).

According to recommendation of many earlier studies, layer by layer condensation technique was used in present study. But transparent matrix and reflective wedge were not used. The utilization of a transparent matrix is not an easy task due to its greater thickness and lack of rigidity and furthermore many studies showed similar results between stainless matrix combined wooden wedge and transparent matrix combined reflective wedge26,27).

Flowable composite, predicted to reduce stress at the tooth-restoration interface, had weak effect in preventing marginal microleakage. The idea behind the utilization of this type of composite is to take advantage of its flow capacitiy in filling all parts of the box, facilitated by the rounded line angles. Another expected advantage is its lower Young's Modulus in comparison with other hydrid composites13,28). This characteristic could contribute to the dissipation of the contraction stresses during the polymerization. Another recent study found that none of three flowable composites were effective in reducing microleakage with gingival margins placed 1.0 mm coronal to CEJ29). In this study, efficacy of flowable composites as intermediate layer to reduce microleakage was mild. Several explanations may be able to account for this result. Flowable composites are widely known to shrink more than condensable composites because of less filler contents. When Stress during polymerization shrinkage goes beyond bonding strength, marginal failure and subsequent microleakage will occur. The utilization of occlusal irradiation could explain the poorer results obtained with this technique. Even though this fact is under argument, occlusal irradiation tends to pull out the composites from the margins, as it shrinks toward the light source and this situation is even worse in deep caries cavities, such as those margins apical to the CEJ.

The use of sandwich-technique implies the use of two different filling materials for one restoration. With the materials used in this study, this also implies the use of two different pretreatment and bonding procedures: one for the resin-modified glass ionomer cement(RMGIC) or compomer and the other for the composites. In this study open sandwich-technique with resin-modified glass ionomer cement resulted in better marginal adaptation on cervical cemental margins as compared with other intermediate layers or composite. The utilization of glass ionomers to microleakage control has been widely studied and compared against several bonding systems. There are many studies demonstrating the superiority of the glass ionomers30,31). Another observation was that RMGIC has superior performance in relation to conventional GIC and compomers32,33). RMGIC reach a chemical maturation far more rapidly than the conventionals, being better able to resist the early polymerization contraction of the composites and also occlusal stresses. However, many clinicians are uncertain about the correct location of the glass ionomer, extended to the external carvosurface margin or held short of it. Recent developments of these groups of materials, it is possible to extend them to the carvosurface margin safely and obtain reliable results34). By Dietrich & others19), extending the RMGIC to external surface ("Open sandwich") produced better results than maintaining it short of the margin ("Closed sandwich"). But even RMGIC could not prevent an extensive microleakage at the cemental margins of composites. This result could be explained by the fact that initial curing shrinkage values of RMGIC are comparable to those of resin composites. However, when bonded to cavity walls the shrinking of the material results in polymerization shrinkage stress can be reduced by compensating mechanisms. The water uptake of resin-modified glass ionomer is substantially higher than that of composites. Furthermore the porosity in hand-mixed RMGIC and higher flow may have contributed to lower shrinkage stress. Even if RMGIC didn't show perfect marginal sealing abilty, cariostatic ability of that material is helpful to clinical condition that cervical margin is on cement. Another advantage of open sandwich technique on class II is the fact that GIC is less sensitive to contamination by saliva and blood which is frequently occured in deep cervical restoration35).

Studies with compomers as intermediate layer in class II composite restorations didn't accomplished widely. In several studies, compomer showed favorable effect on microleakage regardless of statistical significance11,19,23,36). But in this study, compomer showed the worst marginal adaptation, which came short of expectation. This result may be based on the distance from curing light. Curing manner of compomer differs from that of RMGIC which can be cured in condition of no curing light. Compomer can't cure without initial curing light. In one recent study, open sandwich technique with compomer showed good marginal sealing ability before mechanical loading. But after mechanical loading this technique showed worse result than the closed technique and this results were caused by incomplete conversion of resinous monomer to polymer.

In spite of many efforts to eliminate marginal microleakage, complete elimination of the microleakage could never be attained in relation to the cementum/dentin margins. But, the pulp has its own self defense mechanism, such as the production of reparative dentin, sclerosis of dentinal tubules. These reactions could minimize the microleakage effects. Also studies accomplished in laboratory have tendency to overestimate the in vivo microleakage results23). With reference to in vitro results, our effort to search for a technique that reduce the microleakage effects must be continued. In connection with intermediate layer treatment, further works must be focused on method to simplify bonding procedures with two restorative materials. Also, not only microleakage but also anticariogenic effect of intermediate layer materials with fluoride-releasing effect should be compared in further studies.

V. CONCLUSION

Many techniques have been proposed to eliminate the microleakage which was caused by polymerization shrinkage of direct Class II composite restoration. Among those techniques, the effect of intermediate layer treatment prior to placement of condensable composites were evaluated in this study. Forty box-shaped Class II cavities whose cervical margins were placed below CEJ were prepared. Restorative composites and dental adhesive used in this study were Clearfil AP-X and Clearfil SE Bond. Group I was restored without intermediate layer and Group II, III and IV were restored with intermediate layers which were flowable resin, compomer and resin-modified glass ionomer cement respectively.

Results of this study revealed resin-modified glass ionomer as intermediate layer had superior effect (p<0.05) and other intemediate layer had no effect on reducing microleakage(p>0.05).

In the case of Class II direct composite restoration, resin-modified glass ionomer cement restoration to cervical portion which margins below CEJ prior to placement of condensable composites would be advantageous to reduce microleakage.

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