| Abstract|| |
Aim: To compare newer bulk-fill composites with an incrementally filled composite for adaptability and subsequent gap formation at the pulpal floor.
Materials and Methods: Class I cavities were prepared in 60 intact molars, with a shallow depression in the center of the pulpal floor. The samples were divided into four groups (n = 15), according to the material used; smart dentine replacement (SDR), SonicFill, Ever X Flow and Z350 XT, restored to a depth of 4 mm. Following thermocycling, samples were sectioned buccolingually and examined under a stereomicroscope. Seven samples from each group were coated with nail varnish except for approximately 1 mm around the tooth restoration junction. These samples were examined under stereomicroscope after staining with 2% buffered methylene blue dye. The remaining samples were examined under a scanning electron microscope for gap formation. The data were statistically analyzed using one-way ANOVA and post-hoc Bonferroni test.
Results: SDR showed the significantly best adaptability as compared to both SonicFill and Ever X Flow (comparable). However, significantly least adaptive capacity was seen in the incrementally filled group (Z350 XT).
Conclusion: Bulk-fill composites performed better than incremental composites, demonstrating better adaptability and less gap formation at the pulpal floor.
Keywords: Adaptability; bulk-fill composites; gap formation; incrementally filled composites
|How to cite this article:|
Kapoor N, Bahuguna N, Anand S. Influence of composite insertion technique on gap formation. J Conserv Dent 2016;19:77-81
| Introduction|| |
The performance of dental restorations is influenced by several factors, including the restorative materials used, the clinician's level of experience, the type of tooth, the tooth's position in the dental arch, the restoration's design, the restoration's size, the number of restored surfaces, and the patient's age. 
Amalgam was the material of choice worldwide for Class I and Class II restorations for more than a century.  However, rising demand for the use of esthetic materials in posterior teeth has increased dramatically over the past two decades, surpassing amalgam. 
Composites are gaining popularity over amalgam as posterior esthetic restorative material because of the following reasons: Scientific advances in the development of superior alternative restorative materials,  esthetic reasons,  cavity preparation is both less invasive and less extensive, placing amalgam restorations without a dentine-bonding agent fails to seal the margins,  and last but not the least is mercury toxicity. 
However, the main problem faced by composites is polymerization shrinkage and stresses which depends on multiple factors such as the configuration factor, composition of resin composites, material properties, various incremental placement techniques,  and different modes of curing. 
According to Karthick et al.  to overcome this problem, various methods have been employed; the incremental curing technique being one of them. The various incremental techniques used are faciolingual layering (vertical), gingiva-occlusal layering (horizontal), three site technique, wedge-shaped layering (oblique), successive cusp build-up technique, bulk technique, and centripetal build-up. 
However, this technique has many disadvantages: It is difficult to place the multiple increments leading to an increase in the arduousness of the task and the time it takes to complete it. If not performed properly, placing multiple layers can result in polymerization shrinkage and marginal leakage. 
In light of this, a group of new products were recently introduced, known as "bulk-fill composites." These materials are recommended for insertion in a maximum 4-mm bulk due to their high reactivity to light curing. 
The rationale of the bulk-fill resins is to reduce clinical steps by filling the cavity in "single" increment,  leading to a reduced porosity and a uniform consistency for the restoration, further reducing the clinical time taken and cost factor for the patient. 
At present, three types of bulk-fill resins are available, distinguished primarily by their viscosity, which is low, for example, smart dentin replacement SDR (Dentsply, DeTrey, Konstaz, Germany), Venus Bulk Fill (Heraeus Kulzer, U.S.A), medium for eg, Tetric Evoceram Bulk Fill (Ivoclar Vivadent, Amherst, NY), QuixFil (Dentsply, UK), x-Tra Fil (Voco, U.S.A), or fluctuating, for example, SonicFill (Kerr, Sybron Endo, U.S.A) and fiber- reinforced bulk-fill composite, for example, Ever X flow (GC, Europe). 
The null hypothesis of the study was that the adaptability and subsequent gap formation at the pulpal floor do not depend upon the bulk fill or incremental technique used to fill the prepared cavities.
| Materials and Methods|| |
This study was performed at the Institute of Dental Studies and Technologies in the Department of Conservative Dentistry and Endodontics. Sixty intact human molars were selected randomly. Class I cavity to the depth of 4 mm was prepared in each tooth, with a shallow depression in the center of the pulpal floor using a standardized round bur (Mani Inc., Utsunomiya, Tochigi, Japan) of size Ό and 0.5 mm diameter under profuse water cooling. The depth of Class I cavity prepared was 4 mm as per manufacturer's claim that bulk-fill composites can be filled beyond 4 mm. 
The samples were then divided into four groups of 15 teeth each. The teeth were etched using 3M ESPE Scotch bond multi-purpose etchant for 20 s following which the specimens were washed with distilled water for 15-20 s and further blot dried for 20 s. Adper Single Bond adhesive (3M ESPE, St.Paul, MN, USA) was applied to and scrubbed on the surface for 20 s to create a good hybridization of the etched area. Next, the adhesive was air-thinned until the entire carrier was evaporated. It was then light-cured (Bluephase C8 Ivoclar Vivadent, Amherst, NY) for 20 s.
All the cavities prepared were then restored to a depth of 4 mm using different materials:
- Group I: SDR (Dentsply, DeTrey, Konstanz, Germany),
- Group II: SonicFill (Kerr, Sybron Endo, USA),
- Group III: Ever X Flow (GC, Europe), and
- Group IV: Z350 XT (3M, Latin America).
Groups I-III are bulk-fill composites and Group IV includes incrementally filled composite.
Each group was light-cured for 20 s with light emitting diode light curing unit.
Specimens were then subjected to a thermocycling regimen of 2500 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 bath. 
All the samples were sectioned buccolingually and examined under the stereomicroscope at ×25 magnification for evaluation of their adapting capacity at the pulpal floor. Seven samples from each group were coated with two layers of nail varnish except the area of approximately 1 mm around the tooth and restoration junction. Finally, these sections were stained with 2% buffered methylene blue dye for 24 h and examined under the stereomicroscope for dye penetration.
The remaining eight samples from each group were taken to vacuum desiccators and sputter coated with gold-palladium. Each specimen was then examined under a scanning electron microscope (SEM) for gap formation at the pulpal floor.
The obtained results were statistically analyzed using SPSS version 18.0 (SPSS Inc, IBM Corp, Chicago, USA) with one-way ANOVA and post-hoc Bonferroni test for multiple comparisons to compare means of different groups. The differences were considered statistically significant for P < 0.05.
| Results|| |
The results of the study showed that the bulk-fill composites demonstrated better results than incrementally filled composites. [Table 1] summarizes the results (mean, standard deviation, median, minimum, and maximum values) for gap formation observed under SEM [Figure 1]a-d for all the groups.
|Figure 1: Scanning electron microscope images for all the groups showing gap formation are as follows: (a) Smart dentine replacement. (b) SonicFill. (c) Ever X Flow. (d) Z350 XT (3M)|
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|Table 1: Mean, SD, median, minimum, and maximum values of gap formation (mm) observed under SEM in different groups|
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[Table 2] represents the multiple comparisons between different pair of groups for gap formation (μm) by post-hoc Bonferroni test. The significant difference at 0.05 level of significance in gap formation, i.e., P < 0.05 was obtained between all the groups.
|Table 2: Comparison between different pair of groups for gap formation (mm) observed under SEM in different groups (by post-hoc Bonferroni test for multiple comparisons)|
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The stereomicroscopic images [Figure 2]a-h showed that SDR has the best adaptability. However, SonicFill and Ever X Flow showed comparable results. On the other hand, Z350 XT showed the least adaptation.
|Figure 2: Stereomicroscopic images showing the adaptability before and after dye penetration. (a) Smart dentine replacement before dye penetration. (b) SonicFill before dye penetration. (c) Ever X Flow before dye penetration. (d) Z350 XT (3M) before dye penetration. (e) Smart dentine replacement after dye penetration. (f) SonicFill after dye penetration. (g) Ever X Flow after dye penetration. (h) Z350 XT (3M) after dye penetration|
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Out of the bulk-fill composites, SDR (2.1824 μm) showed statistically significant least gap formation followed by SonicFill (2.5178 μm) and Ever X Flow (3.3379 μm), which in turn showed statistically comparable results. Z350 XT (6.1059 μm), on the other hand, also demonstrated significantly more gap formation and the less adaptability as compared to SDR.
| Discussion|| |
Traditional composites were self-cured and prepared as two components to be mixed just before use, resulting in inadvertent air incorporation leaving pores as mechanical defects that were extremely deleterious to strength. 
Then came the incrementally placed composite resins, but the big challenge faced by them was the incorporation of voids or contamination between composite layers, bond failures between increments, difficulty in placement because of limited access in conservative preparations, and the increased time required to place and polymerize each layer. 
Compared to the incremental build-up technique of the restoration, today's bulk-fill composites  require less chair side time and are more predictable,  making the restorative process comfortable for the patient.
In our study, the bulk-filled composites used demonstrated significantly better results than the incremental composites. It could be claimed that bulk-fill composites demonstrated enhanced flowability leading to good adaptation; elasticity and low polymerization shrinkage stress which reduces microleakage, reduced postoperative sensitivity and secondary caries; improved depth of cure of at least 4 mm eliminating the need for layering,  and can be cured in bulk as they are highly translucent which in turn allow the curing light to sufficiently penetrate to the bottom of single increment layer. 
According to Chuang et al.,  SDR, is designed to be used as a posterior restorative material as it can be placed up to 4 mm single increment, shows 60% less polymerization shrinkage and 30-50% reduction in procedure time compared with conventional composites. It has a self-leveling consistency for optimum adaptation to the cavity and is compatible with current adhesive systems. 
In accordance with the results, a study conducted by Van Ende et al.  showed that SDR provided satisfactory bond strengths regardless of the filling technique and cavity depth. However, adhesion failed when conventional composites were used in bulk.
SDR showed the best results in the present study. These results corroborate with a study conducted by Orłowski et al.  in which Class II cavities were restored with four bulk-fill type composites and the highest rating of no dye penetration was achieved with the restorations made of the SDR as compared to the restorations made of SonicFill system.
Rheine  suggested that SDR is the flowable composite which can be used as a bulk-fill base material in increments of up to 4 mm, as also recommended by the manufacturer. The main difference lies in a modulator (on the activated photoactive group) that is incorporated into a urethane-based dimethacrylate which reduces polymerization stress  by slowing the radical polymerization rate. 
Contrary to the result of this study, Poggio et al.  conducted an in vitro study in which high microleakage values were recorded for SDR as compared to SonicFill when "deep" Class II cavities were prepared and restored with gingival cavosurface margin below the cementoenamel junction.
Another bulk-fill composite resin used in this study was SonicFill, a sonic activated bulk-fill system with single increment bulk-fill having a bulk-cure up to 5 mm for expediency, has a high wear resistance for durability, and a reduced translucency for acceptable esthetics. 
In this study, SonicFill showed significantly comparable results to Ever X Flow, but lesser adaptability and increased gap formation as compared to SDR. The SonicFill system overcomes the difficulty by applying external sonic vibrations and temperature by lowering its viscosity at the point of delivery by applying internal vibrations to the resin, without the need for heat or external handheld sonic devices.  It, thus, has adaptability of a flowable and the sculpt ability of a universal composite, obviating the need of an initial lining or a capping occlusal layer. 
These results are supported by Sabbagh J. who stated that SonicFill is a fast and reliable new technique for posterior restorations which does not require any additional capping layer. The manufacturer stated that as sonic energy is applied through the handpiece, the modifier causes the viscosity to drop (up to 87%), increasing the flowability of the composite, enabling quick placement, and precise adaptation to the cavity walls. 
However, Munoz-Viveros et al.  who evaluated the sealing ability of SonicFill composite in a Class II restoration in comparison with SureFil SDR and Filtek Supreme Ultra, concluded that SonicFill has equivalent margin adaptation of Filtek Supreme Ultra filled in 2 mm increments and SureFil SDR which requires an additional overlay material (Ceram X).
A fiber-reinforced composite, Ever X Posterior, specially designed to be used as dentine replacement, in conjunction with a conventional composite was also used in this study showing comparable results to SonicFill, but less than SDR. It consists of a combination of a resin-matrix, randomly orientated short E-glass fibers, and inorganic particulate fillers. The resin matrix contains bisphenol A-glycidyl methacrylate, triethylene glycol dimethacrylate, and polymethyl methacrylate forming a matrix called semi-interpenetrating polymer network (net-poly(methylmethacrylate)-inter-net-poly(bis-glycidyl-A-dimethacrylate)) which provides good bonding properties and improves toughness of the polymer matrix.  The short fibers prevent and arrest crack propagation through the restoration and tooth structure, which is considered to be the main cause of composite failures.
These results suggest that SDR showed the best adaptive capacity and the least gap formation as compared to the other bulk-fill composites used because of its self-leveling property allowing optimal adaptation to the cavity walls.
| Conclusion|| |
Within the limitation of this study, it was concluded that the new bulk-fill composites provided better results than the incrementally filled conventional composites.
It can, thus, be suggested that SDR because of its self-leveling property can be used as a base layer and can be overlaid by SonicFill having adaptability of a flowable and the sculpt ability of a universal composite.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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Dr. Neha Kapoor
Department of Conservative Dentistry and Endodontics, Institute of Dental Studies and Technologies, Modinagar, Ghaziabad, Uttar Pradesh
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2]
[Table 1], [Table 2]