In vitro evaluation of microleakage of class II packable composite resin restorations using flowable composite and resin modified glass ionomers as intermediate layers

Abstract

Aim and Objectives:

To evaluate the cervical marginal microleakage of class II packable composite resin restorations using flowable composite and resin modified glass ionomer as intermediate layers and whether the difference in the thickness of these intermediate layers would influence the microleakage.

Materials and Methods:

Standardized class II box only cavities (4 mm bucco lingual width 2 mm mesio distal depth with the gingival margin 1 mm above the cemento-enamel junction (CEJ) were restored as follows: Group A Restoration with packable composite alone, Group B, Subgroup 1, 1 mm flowable composite liner + packable composite, Sub Group 2, 2 mm flowable composite liner + packable composite, Group C, Subgroup 1, 1 mm resin modified glass ionomer cement (RMGIC) liner + packable composite, Sub Group 2, 2 mm RMGIC liner + packable composite, The specimens were thermocycled, stained with methylene blue, sectioned to evaluate the dye penetration. Data were analyzed using Kruskall Wallis Test and Mann Whitney U test.

Results:

There was no statistically significant difference between the groups. The difference in the thickness of the intermediate layers did not influence the microleakage.

Conclusions:

Use of 1 mm of flowable composite intermediate layer improved the sealing ability of packable composites than the differential thickness of resin modified glass ionomer.

INTRODUCTION

The integrity and durability of the marginal seal is essential for any restorative system to maintain pulpal health and to increase the longevity of the restoration. One of the weak links with class II composite resin restorations is microleakage at the gingival margin of the proximal box which contributes to postoperative sensitivity, high incidence of secondary caries accounting for many clinically failed restorations.[1] Techniques proposed and tested to address this problem include incremental placement, clear matrix and reflecting wedges, directed shrinkage technique, three sited technique, pre cured composite inserts, beta quartz inserts, the use of auto polymerizing composites, pulse stepped curing and sandwich technique incorporating glass ionomer cements or other materials as an intermediate layer.[2] Despite these attempts, substantial microleakage at the gingival margin continues to be reported.

Unlike amalgam, traditional composites do not offer any resistance to placement forces and tend to be sticky resulting in a tendency to be pulled away from the cavity wall when the placement instrument is withdrawn. This difficulty in obtaining intimate cavity adaptation will lead to incomplete marginal sealing.[3]

To counter these problems, packable composites have been developed by densely loading fillers into hybrid composites with improved mechanical properties such as decreased wear, increased packability and increased depth of cure. The reduced polymerization shrinkage achieved through increased filler loading may offer significantly decreased marginal leakage. However, the increased viscosity and modulus of elasticity is thought to prevent complete wetting of the cavity walls during placement. To address this shortcoming, an 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 auto polymerizing composites,[2] flowable resin composites (FRC),[4–6] self cured and resin modified glass ionomer cements (RMGIC).[7,8]

FRC being less viscous material improve the wettability by flowing onto all prepared surfaces creating an intimate union with the microstructural defects in the floor and the walls of the cavity preparation.[4,5] Also act as a flexible intermediate layer that help relieve stresses during polymerization shrinkage of the restorative resin.[6] These characteristics and a syringe delivery system makes them an ideal choice for the use as a liner.[4]

RMGIC have molecular bonding to dentin and enamel, bacteriostatic action, thermal expansion similar to that of enamel and dentin and a slow setting reaction with a low setting shrinkage.[7] The “Open Sandwich” technique has been recommended in high caries risk patients. The improved mechanical and physical properties compared with those of conventional glass ionomer cement (GIC) increase the quality and longevity of open sandwich restorations.[8]

Hence, the purpose of this in vitro study was to evaluate whether the intermediate layers of FRC and RMGIC and their differential thickness would eliminate or significantly decrease microleakage at the gingival margin of class II packable composite restorations.

MATERIALS AND METHODS

Hundred intact extracted molars devoid of caries, restorations, and cracks were chosen for the study. They were stored in normal saline until used. Class II box-only cavities were prepared on either of the proximal surfaces of each tooth (4 mm bucco-lingual width and 2 mm mesio-distal depth) with the gingival cavosurface margin 1 mm above the CEJ with the cavosurface margins as butt joint. Each tooth was mounted in a stone jig with one premolar and one molar on the mesial and distal sides, respectively, to simulate the posterior teeth alignment. The teeth were randomly divided into three major test groups. Prior to restoration, the tooth was wrapped with ivory no 8 matrix band and tofflemire retainer. The wooden wedges were inserted inter proximally in order to tightly seal the cervical margins. A sharp explorer was used to confirm the adaptation of the matrix band to the cervical margin.

RESTORATIVE PROCEDURE

Group A

The cavities were etched (ULTRAETCH, ULTRADENT) bonded (ADPER SINGLE BOND2, 3M ESPE) and restored with packable composite (Filtek P60, 3M ESPE), using the oblique incremental technique with each increment being 2 mm. Each increment was cured for 20 seconds from the occlusal aspect. The restoration was then light cured from the buccal and lingual aspects for 20 seconds each after the matrix band was removed.

Group B

Sub Group 1 (n = 20 teeth/40 restorations)

After etching and bonding, FRC (FILTEK FLOW, 3M ESPE) was injected onto the gingival floor of the cavity to a thickness of approximately 1 mm and light cured for 20 seconds. The thickness was controlled referring to the original cavity depth using a standard William's periodontal probe. The cavity was then restored with packable composite.

Sub Group 2 (n = 20 teeth/40 restorations)

The restorative procedures were similar to group B subgroup 1 with the thickness of the FRC approximately 2 mm.

Group C

Sub Group 1 (n = 20 teeth/40 restorations)

After conditioning the cavity with 10% polyacrylic acid (DENTINE CONDITIONER , GC CORP.) for 10 seconds, RMGIC (VITREBOND, 3M ESPE) was mixed according to the manufacturer's instructions and was placed on the cavity floor to a thickness of approximately 1 mm and light cured. The cavity was then etched, bonded, and restored with packable composite.

Sub Group 2 (n = 20 teeth/40 restorations)

The restorative procedures were similar to group C subgroup 1 with the thickness of the RMGIC approximately 2 mm.

Following completion of the restoration, the restoration was finished and polished with a fine diamond point and a series of abrasive disks. The teeth were the placed in isotonic saline in a water bath at 37°C for 24 hrs and thermocycle tested for 1500 cycles with temperature range of 5°C to 60° ± 5°C with a dwell time of 20 seconds for each temperature.

Interface microleakage evaluation

The root apices of these teeth were sealed with sticky wax and all the surfaces were coated with two layers of nail varnish from beyond 1 mm of the gingival margin of the restorations. The teeth were soaked in 0.1% methylene blue dye in a 37°C water bath for 24 hrs.

After removal from the dye solution, they were thoroughly washed under tap water and sectioned mesiodistally into halves along their long axis using a diamond disc with water coolant and examined at 10X stereomicroscope. The cervical marginal microleakage was recorded based on the following criteria.[9]

Score 0 = No dye penetration

Score 1 = Dye penetration limited to enamel

Score 2 = Dye penetration beyond the dentino enamel junction but limited to 2/3rds of the cervical wall length

Score 3 = Dye penetration beyond 2/3rds of the cervical wall length but not to the pulpal wall

Score 4 = Dye penetration to the pulpal wall

RESULTS

The results were tabulated and evaluated by Kruskall Wallis Test [Table 1] and Mann-Whitney U test [Table 2] for statistical significant differences among the groups.

Table 1.

Data evaluated by kruskall wallis test

Table 2.

Comparison of two group pairs at a time using Mann Whitney U test

The P value indicates that there is homogeneity among the five groups. The difference in the thickness of the intermediate layers did not influence the microleakage scores significantly. To confirm this, pair wise comparison is made using Mann-Whitney U test.

Pair wise comparison using Mann Whitney test, showed that there was no statistically significant difference among the group pairs compared.

DISCUSSION

The results in group A are in agreement with the studies which showed no significant difference in the leakage scores when a packable composite was used alone to when an intermediate layer was used.[3,10,4,11] The low polymerization shrinkage of the packable composite, the incremental technique used for curing the resin, the type of adhesive system used might have improved the marginal seal. Single bond is a water/alcohol based system. Water-containing systems when applied to air dried, shrunken dentin plasticize the collapsed collagen by their water content which may gradually be expanded again at the same time that resin monomers infiltrate. Additionally, single bond is an adhesive that takes the advantage of the polyalkenoic acid copolymer derived from the glass ionomer chemical bonding concept. The polyalkenoic acid copolymer has been reported to form Ca-polyalkenoate complexes at the superficial 3 mm of the dentinal tubules. These complexes might stabilize the bonded interface by providing water stability and a stress relaxing effect.[12]

In group B, 1 mm of FRC liner improved the sealing ability than a 2 mm thick intermediate layer. These results are in agreement with the studies[5,9,13] which showed a reduction in the microleakage when a thin lining was used. But it would be very demanding to confirm to a thickness of 1 mm in a deep proximal box. Verification of intermediate layer thickness in a deep proximal box is more difficult in a clinical situation. The wear rate of FRC is higher than that of the packable composites. Therefore, it has been proposed that FRC should be used in only contact free areas. The FRC have a less filler loading and shrink more when used in greater thickness. The results are also in agreement with a study which showed that restorations with thick FRC lining exhibited reduced perfect margin and the potential risk of marginal degradation.[9] The application of 1 mm FRC is considered clinically acceptable.

In group C, 1 mm of RMGIC as an intermediate layer showed better sealing ability than 2 mm of RMGIC although not statistically significant. The results of the present study are in agreement with a study which concluded that the FRC plus a bonding agent in the proximal box of a class II restoration significantly reduced the microleakage at the cervical margin when compared with an injectable glass ionomer with or without bonding agent.[5] The leakage was attributed to porosities and micro gaps with the glass ionomer due to differences in particle size and viscosity between the FRC and the RMGIC. The same could be the reason in this study. The results of the present study are not in agreement with the studies which showed a reduction in the cervical marginal microleakage when a RMGIC intermediate layer is used.[14–16] This difference is due to the type of composite used for the restoration and the number of thermocycles. In the present study, 1500 thermocycles were done in contrast to 600,[14] 250,[15] and 100 cycles[16] in the other studies. Also Vitrebond shows a volumetric contraction of 2.3% on curing. All these parameters could have affected the marginal microleakage in this study.

Limitations of the study

1. In this study, only vertical sectioning was performed in the mesial-distal direction. It has been suggested that a more accurate way to evaluate the total leakage is to completely remove the restoration and evaluate the total amount of leakage as this can vary from different sections.

2. Mechanical loading was also not done to simulate the intra oral conditions.

3. The present study utilized only materials from one manufacturer which would be difficult to follow in a clinical setting. Since great variability exists in the material composition from one manufacturer to the other, the results cannot be generalized to include other combinations. Further studies would be beneficial in this regard.

CONCLUSIONS

According to the methodology used and within the parameters of this in vitro study, the following conclusions are made

1. There was no significant difference between groups in terms of leakage scores.

2. The difference in the thickness of the intermediate layers did not significantly influence the microleakage.

3. Use of 1 mm of FRC as an intermediate layer showed overall less microleakage when compared with the other groups.

4. The groups were efficient in the following order

   1. Group B subgroup 1
   2. Group C subgroup 1
   3. Group A
   4. Group B subgroup 2
   5. Group C subgroup 2