Evaluation of cervical marginal and internal adaptation using newer bulk fill composites: An in vitro study.

OBJECTIVE: To evaluate the cervical marginal and internal adaptation of posterior bulk fill resin composites of different viscosities, before and after thermo-cycling (TMC).
MATERIALS AND METHODS: Eighty box-only class II cavities were prepared in 40 extracted human premolars with the distal proximal box beneath the enamel-cementum junction (CEJ). The teeth in the experimental groups were restored with bulk fill resin composite restorations (Gr. I- Sonic Fill, Gr. II- SDR, Gr. III- Tetric N Ceram Bulk Fill or a conventional composite designed for 2-mm increments (Gr. IV- Tetric N Flow along with Tetric N Ceram). Before and after thermal cycling, the gap-free marginal length was analyzed using SEM of epoxy resin replicas. After thermal cycling, specimens were cut longitudinally in order to investigate internal dentine adaptation by epoxy replicas under SEM (500 × magnification).
RESULTS: Statistical analysis was performed using the ANOVA and Tukey Post Hoc tests (P < 0.05). In enamel, high percentages of gap-free margins were initially identified for all the groups, which declined after thermal cycling. However, no significant differences were identified among any of the groups (P > 0.05). In dentine, bulk fill groups performed at par with the incremental placement; for both marginal and internal adaptation (P < 0.05), for all materials except Tetric N Ceram Bulk Fill.
CONCLUSIONS: Viscosity of the bulk fill restorative material influenced the proportion of gap-free marginal interface and the internal adaptation in dentin.

INTRODUCTION

The success rate in restoring a class II cavity lesion depends upon the type of dental material used for restoration as well as the skill with which an operator performs the procedure. The clinician's main concern when placing direct, posterior, resin-based composite restorations would be to counter polymerization shrinkage stress and its consequent outcomes.[12]

Polymerization shrinkage causes stress at the tooth-restoration interface as the elastic modulus of the restorative resin increases during light activation. The detrimental effects of polymerization shrinkage stress include bond failure, cuspal flexure, interfacial gap formation and subsequent microleakage.[3] The resulting marginal discoloration is often misdiagnosed as recurrent caries at the margins leading to unnecessary restoration replacement and further tooth tissue loss.[45] Furthermore, the occurrence of recurrent cervical caries in posterior teeth has been reported to be eight times higher than the recurrent decay at occlusal margin, of class II composite restorations.[67]

The incremental layering technique and use of low-modulus intermediate liner material such as flowable composites have been suggested to reduce this shrinkage.[89] However, every dentist desires a posterior composite resin with handling properties similar to dental amalgam.

In this context, a group of products were recently introduced as ‘bulk fill composites’. These materials are recommended for insertion in a 4-mm bulk due to their high reactivity to light curing. However, the potential for development of internal and marginal discrepancies exists with bulk placement and the proportion of gaps relative to use of conventional 2-mm increments needs to be ascertained.[10] If the bulk fill restorative materials are to provide a true clinical advantage, then they require high depths of cure while simultaneously demonstrating a decrease in internal stress, and subsequently enhanced adaptation to the tooth substrate.

The aim of this in vitro study was to investigate the cervical marginal and internal dentinal adaptation among different posterior bulk fill restorative material systems in class II cavities. The research hypotheses tested were:

There would be no significant differences in marginal and internal adaptation at the cervical tooth restoration interface in cavities restored using either the bulk fill or the incremental fill technique.

Bulk fill composites of different viscosities provide the same marginal adaptation quality before and after thermo-cycling (TMC) in restored class II cavities.

MATERIALS AND METHODS

Sample preparation

Forty intact, non-carious, unrestored human maxillary first premolars, extracted for therapeutic reasons were collected for the study. The teeth were hand scaled and kept in 0.05% thymol solution at 37°C for no longer than 1 month before using. The teeth were examined to ensure that they were free of defects under an operating microscope at 20 × magnification.

Eighty box-only class II cavities with parallel walls were prepared, with the distal cervical margin established one mm below the CEJ. The overall dimensions of the cavities were standardized as follows: 5 mm wide bucco-lingually and 2 mm deep axially. The cavities were cut using coarse diamond points under profuse water cooling, and finished with finishing diamond points (one pair of diamond points per four cavities). Inner angles of the cavities were rounded and the enamel margins were not bevelled. The teeth were randomly assigned to the four experimental groups (n = 10). The prepared teeth were mounted in a jig featuring artificial training teeth. A metal matrix band was adopted and a dental probe was used to place marks on the outer surface of the matrix, allowing height control of the subsequently placed increments.

Adhesive procedures were performed with Tetric-N Bond (Ivoclar Vivadent) adhesive system following the manufacturer's instructions. Total-etch dentine bonding system was used in all the groups to reduce variability in results. Three commercial bulk fill composite systems were tested and one conventional composite that required 2-mm increments was used as control. The experimental groups were:

Gr. I — Sonic Fill (Kerr/Sybron Orange, CA); Increment thickness-4mm

Gr. II — SDR (Dentsply, Konstanz, Germany) + Ceram X Mono; Increment- 4mm + 2mm

Gr. III — Tetric N Ceram Bulk Fill (Ivoclar Vivadent); Increment thickness- 4mm

Gr. IV — Tetric N Flo + Tetric N Ceram (Ivoclar Vivadent); Increment- 1mm + 2mm

Sonic Fill composite was inserted by sonic activation using the propriety hand-piece. Each 4-mm increment was light-cured for 40 s while the 2-mm increments were cured for 20s with a LED light curing unit (Bluephase C8, Ivoclar Vivadent) with output irradiance of approximately 800 mW/cm2 held in contact with the coronal edge of the matrix band. After removal of the matrix band, the restorations were light-cured from their buccal and lingual aspects for an additional 20 s on each side. The polishing procedure was conducted under 20 × magnification with flexible disks (SofLex Pop-on, 3 M ESPE, St. Paul, USA).

Quantitative marginal SEM analysis

After finishing procedures, impressions of the tooth-restoration interface were made using a polyvinyl-siloxane material (Virtual light body, Ivoclar Vivadent AG, FL Schaan, Liechtenstein) and epoxy resin (Diemet-E, Erkodent, Germany) replicas were obtained.

TMC was carried out 24 h after the restorative procedure. During this period, the teeth were stored in water at 37° ± 8°C. All specimens were submitted to 2,500 thermal cycles by alternating immersion in water at + 5° ± 8°C and + 55° ± 8°C with a dwell time of 2 min and transfer time of 5 s in each path. After TMC, a new set of epoxy resin replicas was obtained. Scanning electron microscopy (SEM) evaluation (JEOL, JSM 6510 SEM) was carried out at a 500 × magnification.

After evaluating the marginal adaptation, the teeth were embedded in a slow self-curing acrylic resin material and sectioned mesio-distally up to the CEJ. Only one of the two coronal segments was utilized for investigating the internal dentinal adaptation.

The sections were polished and etched for 2 min using a 37% phosphoric acid etching gel (Total Etch, Ivoclar Vivadent), rinsed and dried. Impressions were taken and epoxy resin replicas were obtained. Drying and impression processes were carried out carefully to limit dentine dehydration.

Image analysis

Results for the marginal adaptation (gap-free margin), before and after TMC were expressed as a percentage of the entire margin length in enamel and dentine. The composite/tooth interface was divided into three regions and measurements of marginal gap widths in each region were made at four points at 500 × magnification. The largest marginal gap width in each region was recorded in micrometers (μm), and the mean gap widths for each of the enamel and dentin margins were calculated. Similarly, the internal adaptation to dentin was evaluated.

For the dependent variable “material” statistical evaluation was performed using the Minitab version 5.0 software with one-way ANOVA and Tukey's post hoc test at a 5% level of significance. The differences between marginal quality, before and after TMC were assessed with a paired t-test.

RESULTS

Table 1 demonstrates the values of marginal and internal adaptation expressed as percentage of gap-free continuous margin (% CM). Analysis of variance (ANOVA) test was performed and showed that there were no significant differences when considering the cervical enamel margin between the groups (P = 0.900 before and P = 0.739 after TMC) [Table 3]. For dentine interface the differences in gap-free margin extent were found between groups depending on the bulk fill type (P = 0.000 before and after TMC). The lowest % CM was observed in Gr. III (Tetric N Bulk Fill). The Tukey's post hoc test was performed to confirm the results of ANOVA test between each group for different interfaces. With respect to the cervical dentin interface, there was no significant difference between Group I, Group II and Group IV (control group) (P ≥ 0.05). Both before and after TMC; Group III (Tetric N Bulk Fill) showed a very highly significant difference (P ≤ 0.001) with Group I (Sonic Fill) and Group IV (Control). A highly significant difference (P = 0.008) was noted between group II and Group III before TMC which changed to a very highly significant difference (P = 0.000) after TMC [Table 4]. Also for internal adaptation to dentine, Group III exhibited significantly less areas of gap-free transition compared to other restorative systems (P < 0.05) [Tables 1 and 3]. Mean values and standard deviation for marginal gap values for all the experimental groups are presented in Table 2. Statistically significant difference in gap width at cervical dentin margin was evident only after TMC (P = 0.042) [Tables 3 and 4]. Intra-group comparison showed that gap free margin length deteriorated for all the restorative materials after thermo-cycling; the difference being statistically significant (P< 0.05) [Table 5]. Group I (Sonic Fill) was the only material where the marginal gap width did not show significant variation before and after TMC (P = 0.115 and P = 0.069 for cervical enamel and dentin interfaces, respectively. Representative SEM micrographs are shown in Figures 1-2.

Table 1

Results of marginal and internal adaptation at the cervical tooth-restoration interface expressed as percentages of continuous margins %CM (mean) ± SD before and after thermo-cycling (TMC)

Table 3

ANOVA test results for the experimental groups before and after thermo-cycling (TMC)

Table 4

Tukey's Post Hoc test for comparison between the experimental groups

Table 2

Results of marginal gap values at the cervical tooth-restoration interface expressed in micro-meter (μm) before and after thermo-cycling (TMC)

Table 5

Paired t test results for comparison within each group before and after thermo-cycling (TMC)

Figure 1

Representative SEM (500 ×) micrograph of margins in cervical enamel. (a) continuous margin, (b) Noncontinuous margin

Figure 2

Representative SEM (500 ×) micrograph of noncontinuous margins in cervical dentine. (a) continuous margin, (b) Non-continuous margin

DISCUSSION

The recent introduction of “bulk-filled” restorative materials has reignited the debate of “bulk vs. incrementally” placed composites as the effect of shrinkage stress may be more pronounced with bulk fill since the entire mass polymerizes at one time rather than in small increments.

An ideal bulk fill composite would be one that could be placed into a preparation having a high C-factor design and still exhibit very little polymerization shrinkage stress, while maintaining a high degree of cure throughout.[11]

Currently, bulk fill materials are available in different viscosities, which is low, variable or medium. The present study investigated whether bulk fill composites of different viscosities provide the same marginal and internal adaptation quality when used to restore class II cavities.

In all the experimental groups, mesial class II cavity was limited above CEJ and the distal proximal box was maintained one mm below the CEJ, where in the challenge to dentin adaptation could be analyzed. The results of this study demonstrated that all materials under investigation exhibited satisfactory marginal adaptation before TMC, particularly in enamel. Unfortunately, the level of marginal adaptation was not maintained after TMC. The values of continuous margins in enamel were lower than previous investigations.[12] This could be attributed to the fact that the enamel margins were not bevelled and it has been reported that cervical marginal adaptation is inferior to that observed in proximal and occlusal enamel margins.[13] However, the results reinforce the fact that phosphoric acid etching remains the most reliable mode of pre-treatment to achieve fatigue resistant enamel bonds.

The lower percentage of gap-free margins in dentine gives an indication of an increased potential for gap formation and microleakage. The inferior adaptation to dentine observed in Group III Tetric N Ceram Bulk Fill when compared with the other experimental groups could be attributed to the restricted flow of the material in the cavity.

Tetric N Ceram Bulk Fill represents the medium viscosity type bulk fill. The curing depth of 4 mm is achieved mainly due to the patented photo-initiator, Ivocerin, which is far more reactive than conventional initiators.[14] The stiffer composites help in restoring good contacts in posterior teeth; however, they may not adequately adapt to internal areas and cavosurface margins at the cervical joint.[15]

SDR (Smart Dentin Replacement) (Dentsply, Konstanz, Germany) was introduced to the market as flowable bulk fill composite which incorporates a new stress decreasing resin technology. However, it requires a conventional posterior composite to be cured on top of the 4-mm thick flowable base.[16]

A novel resin composite system, Sonic Fill System (Kerr/Kavo), was recently introduced. The bulk placement is facilitated by a specialized hand-piece, which delivers sonic energy at varying intensities. As sonic energy is applied through the hand-piece, the incorporated modifier causes the viscosity to drop (up to 87%), during the composite insertion. When the sonic energy is stopped, the composite returns to a more viscous, non-slumping state that is more suitable for carving and contouring.[17]

The low viscosity of SDR and Sonic Fill system, which facilitates plastic flow during the early phases of polymerization could be responsible for the better adaptation exhibited by these restorative materials.[18]

The use of flowable resin liners with a low modulus of elasticity under composites as the first increment has become increasingly accepted over the past few years.[1819] They serve as a stress-absorbing layer during polymerization shrinkage of the subsequent increment and act by reducing the effect of the C-factor.[1920] However, the benefit of using flowable composites as gingival increment for reducing polymerization contraction stress is still a matter of controversy.[212223]

Considering bulk fill placement technique, it has been demonstrated that Surefil SDR showed better internal adaptation than conventional composites in high C-factor cavities.[24] Another study showed similar levels of microleakage of bulk fill (Surefil SDR and X-tra Base) and standard (GrandioSO, VOCO) composites.[25]

This study infers that the critical issue regarding internal and marginal gap formation is still a matter of concern. Though composite may be cured to enhanced depths, the potential for developing any post-insertion sensitivity that is related to gap formation at the pulpal floor, and the resulting hydraulic movement of fluids that will occupy this space upon occlusal loading is still present.

CONCLUSION

Within the limitations of this in vitro study, it can be concluded that:

Both bulk fill restorative materials and incrementally layered composite resulted in a similar proportion of gap-free, marginal interface in enamel.

All the experimental groups except for Tetric N Ceram Bulk Fill demonstrated similar dentin adaptability when compared with the control group. Thus, the viscosity of the bulk fill restorative material influenced the proportion of gap-free marginal interface and the internal adaptation in dentine.