Microleakage at the Cervical Margins of Composite Class II Cavities with Different Restorative Techniques - An In-vitro study

INTRODUCTION

Tooth colored posterior composite resin restorations are becoming popular alternatives to amalgam restorations, as they are mercury free, thermally nonconductive, and match the shades of the natural teeth¹. The disadvantage is the failure due to micro leakage particularly at the gingival margins.²

Many techniques have been proposed and tested to address the micro leakage problem in class II restorations³, but none have gained universal acceptance mainly due to their variable results.

With the advent of a new class of composites, the Flowables, many authors⁴ have suggested using them as the first increment in direct class II composites, as they have a low modulus of elasticity that could resist the flexion at cervical regions.

So the present study was undertaken to evaluate and compare in-vitro the marginal leakage and the sealing ability at the gingival wall of Class II slot cavities for composite restorations with margins in enamel and cementum, restored using the commonly advocated 3-sited technique, Resin Modified Glass-Ionomer cement as the first increment and with the recently introduced flowable composite as the first increment.

MATERIALS AND METHODS

A total of 30 human extracted molars were selected for this study. The teeth were cleaned, polished, rinsed in tap water and stored in normal saline.

Conservative Class II slots were prepared on both the proximal sides of each tooth. In one proximal, the cervical margin was positioned 1 mm beyond the cemento- enamel junction while in the other the cervical margin was left 1 mm short of cemento enamel junction, on enamel. The enamel 5 margins gingivally received a 0.5-1mm bevel. A restoration template was fabricated to closely simulate the clinical situation during restoration placement. The natural teeth were mounted on a commercial Dentoform using polyvinyl siloxane rubber base material as template. The teeth were grouped according to the following insertion techniques:  

Group 1 : 3-sited incremental technique.

Group 2 : Resin modified glass ionomer as gingival increment.

Group 3 : Flowable composite as the gingival increment. 

Group I: The teeth were etched and two coats of 3M single bond adhesive dentin bonding agent was applied and cured on the cavity walls. The clear matrix of Hawe Super Mat Matrix system and a light reflective wedge were inserted. A layer of approximately 2mm of packable composite (3M Filtek P60 A ) was carefully packed on the gingival floor of the 3 preparation, cured through the wedges for 40 seconds and then from the occlusal for 20 seconds. A second increment of composite was placed vertically in the lingual portion of the box filling approximately two thirds of its width, carefully avoiding contact with the buccal wall. Then it was cured through the lingual for 40 seconds and 20 additional seconds from the occlusal. The remainder of the box was restored and cured in the same way as for the lingual increment. 

Group II: A conditioner was applied for 10 seconds and then the teeth were rinsed for 10 secs with water and dried. A palodent matrix ((Darway, San Mateo, CA, 94401) was inserted and stabilized with a wooden wedge, and then resin modified glass ionomer, (vitrebond, 3M) liner was mixed, taken on a ball burnisher and placed on the floor of the gingival wall, forming an increment of approximately 2 mm. A vibration was done with the instrument to maximize the adaptation of the resin modified glass ionomer to the floor of the box and to minimize porosities. This layer was light cured for 20 seconds. After etching of the walls with the etchant singlebond adhesive (3M) was applied to the remaining cavity and the packable composite was placed in vertical increments similar to Group I.

Group III: After etching and bonding of the cavity a palodent matrix was applied and stabilized with a wooden wedge. A layer of flowable composite (Filtek flow 3M) was injected on to the floor of the gingival wall with a thickness of approximately 2 mm and vibrated with the metallic tip to make the thixotropic material flow along the inferior portion of the box. Then this increment was cured from the occlusal for 60 seconds. After that, the vertical increments were inserted as previously described for the other groups.

All the specimens were finished, polished, thermocycled and then immersed in 2% methylene blue solution for 24 hrs. After sectioning both hemi sections of each tooth were evaluated at 20 x magnifications with a stereomicroscope for penetration of the dye at the cervical margin. The extension of the dye penetration was evaluated based on the following ordinal scale, independent of its location, below or above the cement-enamel junction.

O = No leakage

1= Light: Leakage extending to half of the cervical wall.

2= Moderate: Leakage to the full extension of the cervical wall, but not including the axial wall.

3= Severe: Leakage to the full extension of the cervical wall and including the axial wall. 

RESULTS

For the purpose of dye penetration analysis, only the gingival floor of the tooth/restoration interface was considered. The results were tabulated and statistically analyzed using the Kruskal-Wallis a non-parametric variance analysis which showed that the values of marginal leakage in enamel and cementum margins were not statistically different for any groups, regardless of the restorative technique employed. A Mann-Whitney U test was carried out to identify to the differences between groups at a level of significance of 5%. It showed statistically significant difference when comparisons were made between the margins located in enamel against cementum across the e-scores of 0 vs 0, the latter demonstrating significantly more leakage. (Table1, Graph1).

DISCUSSION

In the present study it was seen that enamel margins demonstrated a very good seal in contrast to the cementum margins, which is in agreement with the various other studies⁶.

The good seal at enamel margins confirm the effectiveness of the enamel etching technique in controlling the micro leakage in the gingival wall of a Class II composite restoration.⁵ One of the reasons that could explain the good results in enamel is the beveling procedure which improves the marginal seal, mainly due to a better etching pattern on transversely cut rods.⁷

The specimen of Group I restored using the 3-sited technique showed good results in the enamel margin⁸. The advantage of utilizing transparent light reflecting wedges and incremental placement of composite is that the light reflecting wedges favorably direct polymerization shrinkage towards the gingival margin i.e., the polymerization shrinkage vectors are directed more precisely at right angles towards the gingival floor of the proximal box, and also cause the attenuation of the light intensity (transdental polymerization) thus decreasing the contraction stresses and improving the  adaptation.⁹

When the margins were located below the cemento-enamel junction, all the specimens showed moderate to severe leakage. This can be explained based on the studies which contested the theory of contraction of composites toward light on which the idea of 3-sited technique is based, and showed that the direction of the polymerization contraction is more influenced by the quality of adhesion and by the configuration of the cavity than by the position of the light source. ⁵

The existence of an outer layer of 200-300 µm thick at the cemento enamel junction margin, not identifiable as sound dentin and covered by a thin layer of cementum has been described. It did not form a well-defined hybrid layer and the presence of resin tags was rare. This layer was considered hypo-mineralized and hyper-organic, and this may affect quality of bonding and could explain the moderate to severe leakage seen with the technique^1,4,9 .

The specimens in the Group II (Figure 1) were restored with a resin modified glass ionomer as the gingival increment. Studies have showed that with the recent developments of these materials, it is possible to extend them to the cavosurface margin safely and obtain reliable results, so the Open Sandwich technique was followed in this study^9,10,11 .

The better performance of the Resin modified glass ionomer in the enamel margins is believed to be attributed to the instant set of the material, which attains high immediate adhesion that is capable of resisting the contraction force generated by the curing of composite resin^12,13. This stronger immediate bonding minimizes the formation of contraction gaps and reduces the likelihood of micro leakage.¹⁴ There is a residual “flexibility” of glass ionomer liners after curing, resulting in the absorption of the contraction stress of resin composite upon polymerization. The shear bond strength of resin modified glass ionomer to dentin was determined to be 9.3 MPA. In spite of the lower bond strength, the failure mode in shear adhesion studies was cohesive in the ionomer or dentin, leaving the bond intact. It is therefore hypothesized that glass ionomer, because of chemical bond characteristics, could better control micro leakage.

The use of glass-ionomer as a liner in Class II resin composite restorations is advantageous. The benefits of combining the two materials are superior esthetics, strength, and wear resistance of the resin composite restorations and the possible exchange or release of fluorides by certain glass-ionomers. This is of particular significance in Class II situations since the sub gingival proximal area is particularly prone to recurrent caries¹⁵ .

The results from this in-vitro study indicate that regardless of the technique used, a complete seal was not attainable at the cementum margin. Even the Resin modified glass ionomer could not prevent leakage at the cementum margin in this study. The interpretation for this lack of adaptation is difficult but could be the result of similar phenomenon that occurs with composites and polymerization shrinkage. Miyasaki and others measured the volumetric shrinkage of Resin modified glass ionomer and obtained a value of 3% which was higher than value for composite resin control (2.4%) and this contraction could have disrupted the bond at cervical cementum margin. However, the laminate technique employing vitrebond glass-ionomer as a liner minimized the extent of leakage onto the gingival floor. Though not statistically significant, 50% (5 out of 10 specimen in Group II) of teeth restored with resin modified glass ionomer showed no leakage as compared to 30% (3 out of 10 specimen in Group I) in 3-sited technique and 0% (0 out of 10 specimen in Group III) in flowable group .(Graph 1).

The specimens of the Group III (Figure 2) in the study were restored with the flowable composite as the gingival increment. Flowable resin materials have been suggested as liner beneath packable composite assuming that the low-viscosity material will better fill irregular internal surfaces and proximal boxes, thereby improve final marginal integrity and form a stress absorbing layer³. In the present study it was seen that there was a tendency, although not statistically significant, for more microleakage in the flowable composite group in both enamel and cementum margins, similar results were seen in other studies also¹⁴. The Flowable composites contain more resin matrix than traditional hybrid composites, which are more densely filled. Labella et al revealed that flowable composites generally had a higher polymerization shrinkage volume, which ranged from 3.6 to 6.0%, while conventional composites had 1.9 to 2.3% volumetric shrinkage. Increased volumetric shrinkage may indicate the potential for higher contraction stresses at the restorative interfaces as well as the likelihood of bond failure^14,16 . . This and the utilization of occlusal irradiation could explain the poorer results obtained with this technique. Due to the curing from the occlusal, the distance of the increment to the light is more, leading to subpolymerization of the cervical increment, mainly at its inner part, resulting in poor adhesion and impaired physical properties due to less than ideal polymerization of the resin monomers. This situation is even worse in deep cavities, such as those with margins apical to the cement-enamel junction. 

CONCLUSION

From the present study it can be concluded that when the cervical margins were located in the enamel all the techniques demonstrated a very good seal, with little or no micro leakage. When the margins were located below the cement-enamel junction in cementum or dentin, all the techniques showed moderate to severe leakage. Of the three restorative techniques compared, the Resin modified glass ionomer as the first increment provided a superior seal while the flowable composite exhibited a poor seal at the margins. However the differences were not statistically significant. Further in vivo studies are needed to evaluate the long-term seal ability of these materials.