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INTRODUCTION
The clinical longevity of tooth restoration -whether directly or indirectly using composites- greatly depends on the quality and stability of the marginal adaptation. Even today, dental restoration failure is a major complication in everyday dental practice.1,2
The most common reasons for composite restorative replacement include tooth fracture, micro-gap formation with rupture of adhesive bonds, and, consequently, marginal microleakage and secondary caries.3,4 All of these are related to the polymerization shrinkage of the composite. Polymerization of dimethacrylate-based composites is accompanied by substantial volumetric shrinkage ranging from 1 % to 3 %.5
The magnitude of the stress generated by polymerization shrinkage depends on several factors, including the composite modulus of elasticity, molecule size, the relationship between the filler volumetric percentage and organic matrix percentage, methacrylate groups, extension, depth and speed of polymerization and the cavity configuration factor (C-factor), which has a direct relationship with the capacity to release the stresses generated during polymerization. Thus, higher C-factor values correspond to lower bond strengths due to the greater stress generated in the tooth structure at the bond interface.6
To reduce the effects of polymerization shrinkage and internal/ marginal gap formation, modifications in material composition have been suggested, such as modified methacrylate organic matrixes7 and higher photo-initiator concentrations using restoration techniques.
The incremental layering technique is the standard protocol to prevent gap formation due to polymerization stresses and to keep the resin composite bonded to the dental tissue. Unfortunately, this technique requires more attention to detail during the placement of each layer in extended or deep cavities, and it carries an implicit risk of incorporating impurities or air bubbles between the layers. Furthermore, higher thicknesses (up to 2 mm) can result in a poor degree of conversion of monomers, and inadequate polymerization may compromise mechanical properties of composite All of this increases the required treatment time when compared with other techniques.8
For these reasons, today, indirect resin composite restoration (IRC) constitutes an option of contemporary restorative treatment. IRC involves fabricating the restoration outside the oral cavity using an impression of the prepared tooth. This technique overcomes some of the disadvantages associated with direct resin composites, such as polymerization shrinkage to the width of the luting gap. Furthermore, it provides better physical and mechanical properties, ideal occlusal morphology, proximal contouring and wear compatibility with opposing natural dentition. However, this technique is more time consuming and requires extra cost and appointments that may, in turn, be out of patient wishes and Budget.1
In order to overcome the shortcomings of incremental filing technique with conventional composites and eliminate extra cost and additional steps with IRC, some manufacturers have re-introduced resin composites for specific use with the bulk filling technique. Manufacturers claim that flowable resin composite can be placed in bulk (up to 4 mm thickness) and be efficiently photopolimerized to maintain low polymerization shrinkage stress at the same time.9
Although these flowable bulk-fill materials were developed to achieve better sealing of cavity margins, controversial results in terms of marginal properties, marginal leakage, marginal integrity, or gaps are reported in the literature,10,11 and this controversy extends to IRC. Thus, the aim of this in vitro study was to determine whether different types of restoration techniques (i.e., incremental filling vs. bulk-fill vs. indirect resin composite restoration) affected the resin-dentin bond strength and marginal integrity of class II cavities. The null hypothesis was that the bond strength and marginal integrity would not be affected by the type of restoration technique used.
METHODS
An experimental in vitro investigation was made
Tooth preparation and experimental group
The sample were forty-five human maxillary premolars without caries that were extracted and selected to random. The teeth were collected after the patients provided their informed consent. The University Ethics Committee approved this study under protocol number 813.512. The teeth were disinfected in 0.1 % thymol, stored in distilled water, and used within 3 months after extraction. The maximum buccal-palatal width of each tooth was measured with a digital caliper (Absolute Digimatic, Mitutoyo; Tokyo, Japan) prior to inclusion. All teeth were individually mounted in a polyvinyl chloride (PVC) ring filled with acrylic resin (Aut Clear, DentBras; Pirassununga, SP, Brazil) up to 1.0 mm below the cement enamel junction.12
Teeth were then divided into groups according to the type of restoration technique used (i.e., G1: Incremental filling; G2: bulk-fill; G3: indirect resin composite).
Restorative procedure
Standardized class II cavities were prepared in all teeth. The depth of the occlusal box used with these preparations was 5 mm, and the mesio-distal length at the bottom of the proximal box was 3 mm. The depth of the proximal box (mesially and distally) was 6 mm, and it had margins located 1 mm below the cement-enamel junction. The internal walls of each cavity were perpendicular to the top and bottom surfaces, and they had round angles defined by the bur’s shape. The cavities were prepared using a diamond bur under water cooling (#4103, KG Sorensen; Barueri, SP, Brazil), and the margins were not beveled.12
After this stage, the teeth were divided into groups according to the following criteria:
G1: The medium-viscosity composite (Spectrum TPH3, Dentsply, De trey) was applied in a horizontal layer with a thickness of 1.5 to 2 mm. Each increment was separately light cured for 20 s; for each, the light source made contact with the coronal edge of the matrix band.
-G2: The flowable bulk-fill resin composite (Surefill SDR flow resin composite, Dentsply De Trey) was applied in a 3.5- to 4 mm layer and then light cured. Subsequently, the conventional composite was placed in a horizontal layer with a thickness of 1 to 1.5 mm. Each increment was separately light cured for 20 s; for each, the light source made contact with the coronal edge of the matrix band.
-G3: Impressions were made from each cavity preparation using silicone (Express XT - 3M ESPE St. Paul, USA) to produce stone dies (Durone, Caulk/Dentsply) that were used to prepare the indirect composite restoration. The restorations were built from a composite resin (Spectrum TPH3, Dentsply, De trey) that was applied in a horizontal layer that was 1.5- to 2 mm thick. Each increment was separately light cured for 20 s; for each, the light source made contact with the coronal edge of the matrix band. After completing the restoration, the marginal adaptation was checked. Finishing and polishing were completed with flexible disks (SofLex Pop-on, 3M ESPE; St Paul, MN, USA).
In all cavities, the two-step etch-and-rinse adhesive XP Bond (Dentsply DeTrey, Konstanz, Germany) was applied according to the manufacturer’s instructions (table 1), and the cavities were light cured with an LED light for 20 s at 1200 W/cm2 (Radii-cal, SDI; Bayswater, Victoria, Australia). For indirect restoration, luting was performed with Enforce dual-resin cement (Dentsply, De trey). The indirect restorations were maintained in place, and the excess cements were removed with scalers before light curing for 40 seconds in each of the dental surfaces.
Table 1 - Division of groups, restorative technique and restorative procedures
| Group | Restorative Technique | Restorative Procedures | Increments |
|---|---|---|---|
| G1 | Direct | TPH Resin (Dentsply) | Increments of 2 mm each |
| G2 | Direct |
|
Increment of up to 4 mm |
| (Dentsply) | - | ||
| TPH Resin (Dentsply) | Increment of up to 2 mm | ||
| G3 | Indirect | TPH Resin (Dentsply) | Increments of 2 mm each |
After 24 h in distilled water at 37 °C, the proximal margins of all restored teeth were finished with flexible disks (SofLex Pop-on, 3M ESPE; St Paul, MN, USA). A single operator carried out all bonding and restorative procedures in an environment with controlled temperature and humidity.
Microtensile bond strength (μTBS) test
Forty-five restorations (n = 15 teeth per experimental condition) were longitudinally sectioned in both the “x” and “y” directions across the bonded interface with a diamond saw in a Labcut 1010 machine (Extec; Enfield, CT, USA) under water cooling at 300 rpm. This was performed in order to obtain resin-dentin sticks from the cavity floor with a rectangular cross-sectional area of approximately 0.8 mm2. The number of premature failures per tooth during specimen preparation was recorded. The cross-sectional area of each stick was measured with a digital caliper to the nearest 0.01 mm and recorded for subsequent calculation of the μTBS (Absolute Digimatic, Mitutoyo).
Each stick was attached to a modified device for μTBS testing with cyanoacrylate resin (Super Bonder, Loctite; São Paulo, SP, Brazil) and subjected to a tensile force in a universal testing machine (Kratos; São Paulo, SP, Brazil) at a crosshead speed of 0.5 mm/min. The failure mode was evaluated at 40X (HMV-2, Shimadzu; Tokyo, Japan) and classified as cohesive in dentin (failure exclusively within dentin, CD); cohesive in resin (failure exclusively within resin, CR); adhesive (failure at the resin/dentin interface, A); or mixed (failure at the resin/dentin interface that included cohesive failure of the neighboring substrates, M).
Marginal integrity
Impressions of the mesial and distal surfaces of 45 restorations (n = 15 teeth per experimental condition) were then taken with a low-viscosity vinyl polysiloxane material (Express, 3M ESPE). These impressions were used for preparation of replicas in epoxy resin (Epofix, Struers; Rødovre, Denmark). Replicas were coated with platinum (MED 020, Bal-Tec; Balzers, Liechtenstein) for analysis using a scanning electron microscope (Stereo Scam/ LEO; Cambridge, UK). For quantitative margin evaluation, the adhesive interface was observed under 400X magnification. On each proximal surface of the restoration, the interface was divided into 15 areas for conservative preparation and 21 areas for extended preparation (Fig. 1). (13 Each area received a score according to the gap presence: 0 = no gaps observed; 1 = presence of at least one gap/irregularity. The evaluation was performed by a technician under blinded conditions. The marginal integrity was expressed as a percentage of the entire margin length using Adobe Photoshop CC 2014 software (Adobe Systems; Mountain View California, CA, USA).
Statistical analysis
For μTBS and marginal integrity, the experimental unit in the current study was the tooth. The μTBS values of all sticks from the same tooth were averaged for statistical purposes. Similarly, the marginal integrity values of the two proximal surfaces from the same tooth were averaged for statistical purposes. The μTBS (MPa) marginal integrity (%) data were subjected to one-way ANOVA. Tukey’s post-hoc test was used (a = 0.05) using the Statistica for Windows software (StatSoft; Tulsa, OK, USA).
All procedures were developed with high degree of seriously and medical ethic agree with the kind of study. Ethical certifications weren’t necessary because research was in vitro.
RESULTS
Approximately 5 to 6 sticks were obtained per tooth, including those with premature failures. Most of the specimens (80.8 to 100 %) showed adhesive/mixed failures (table 2). For all experimental conditions, no significant difference was observed between the groups (table 3; p> 0.05)
Table 2- Percentage of adapted margin of the restorations
| Group | Mean (%) | Standard Deviation |
|---|---|---|
| G1 | 91.02 A | 9.26 |
| G2 | 86.76 A | 14.78 |
| G3 | 83.98 A | 8.41 |
Means with the same letter are not significantly different from each other (Tukey test, p> 0.05).
Table 3- Mean and standard deviation of shear bond strength in Mpa
| Groups | Mean (MPa) | Standard Deviation |
|---|---|---|
| G1 | 21.77 A | 8.15 |
| G2 | 21.07 A | 8.79 |
| G3 | 19.73 A | 7.55 |
Means with the same letter are not significantly different from each other (Tukey test, p> 0.05).
No significant difference was observed in terms of marginal integrity among the restoration techniques used (table 3; p> 0.05). Representative images of restoration technique groups (Fig. 1 and 2) show adequate, good or excellent marginal integrities obtained with the different restorative techniques.
Fig. 1- Image from Scanning Electronic Microscopy (MEV 30X) of the proximal surface of the restored tooth.
DISCUSSION
Bulk filling is highly desired in routine restorative practice, but concerns about shrinkage stress have prompted hesitation among practitioners. In this study, the decision was made to employ a μTBS and marginal integrity test to evaluate the potential impact of bulk filling on the bond strength of class II cavity dentin when compared with incremental filling and indirect resin composite restoration techniques.
It was assumed that polymerization shrinkage stress would impose tensile stress on the adhesive interface at the bottom of the cavity and thus affect the bond strength and marginal integrity of restorations, especially when adhesive procedures are performed in a high C- factor cavity, as was the case in our study. Contrary to previous studies that evaluated the resin-dentin bond strength on a flat dentin surface, the present research examined bond strength values from a constrained surface. This is important because previous studies found that the bond strength of flat cavities is usually higher than that measured in constrained, high C-factor cavities.6
In the present study, there were no significant differences between the values of μTBS obtained using different restorative techniques (direct and indirect). There were also no significant differences between the means of the μTBS values using different resin composites. This is similar to the results of de Assis et al. (2016),13 where the use of composite bulk-fill Surefill SDR flow was favorable.
The use of this material did not jeopardize the resin-dentin bond strength to the bottom of the cavity. A high degree of conversion for Surefil SDR has been reported when used in layers up to 4 mm thick.11 Additionally, this material also generates less polymerization shrinkage and shrinkage stress, causing less cuspal deflection when compared to a conventional composite applied incrementally.14,15 This composite showed 60 % less polymerization shrinkage. The main difference lies in a modulator (on the activated photoactive group) that is incorporated into a urethane-based dimethacrylate, which reduces polymerization stress, forming a more flexible polymer network.16
Regarding marginal integrity, the results of the present study showed a high percentage of gap-free margins, regardless of the restorative technique used, which is in accordance with previous data.10,13,14 We hypothesize that the favorable sealing quality of restorations, which is independent of restorative technique, can be correlated to the preparation, adhesion technique, cementation, and finishing procedures adopted for the same operator. Thus, we accept the null hypothesis of the study.
The similar marginal integrity obtained with direct and indirect restorations should be considered when selecting a restorative material. In addition, the amount of dentin structure removed, the complexity of the technique, and the cost of treatment should also be taken into account; in this case, direct restoration is advantageous.
It is important to note that the current study did not use all of the commercially available bulk-fill materials. However, the results obtained in our study are in line with one recent 3-year clinical evaluations using posterior teeth. In that study, the authors compared the bulk-filling technique using Surefill SDR flow plus a capping layer made of conventional resin composite with a conventional resin composite applied in an incremental technique.17
The reduction of the required clinical procedure duration is very attractive for simplification; thus, the use of bulk-fill resin composites is an alternative for reducing procedural duration and cost when compared IRC restoration. However, more studies are still needed to clarify the stability of the restoration-dentine interface in the long term.
