Comparison of Microleakage of Self-etch and Total-etch Bonding Agents in Primary Molar Class II Composite Restorations

Introduction

Resin-based materials have been used in dentistry for almost 60 years (1). These materials are acceptable because of their high efficiency in cosmetic dentistry(2). In early 1980s, new clinical data suggested that use of composite resins in posterior teeth did not result in acceptable outcomes due to insufficient physical characteristics in the posterior restorations (3). First-generation resins did not have favorable mechanical properties and experienced complications such as lack of dimensional stability and low elastic modulus, microleakage, wear, micro and macroscopic fractures (2, 4). In recent years, much efforts have been made to fabricate composites with physical and chemical properties similar to the tooth structure. Recent developments have focused on reducing polymerization shrinkage, improving physical properties, abrasion resistance and biocompatibility(5). Flowable composites have low viscosity and contain 20-25% less filler compared to conventional composites and are able to wet and adapt with the cavity surfaces quickly. Following the recent improvements in material characteristics, more recent generations of flowable composites include higher volume of fillers, which provides higher mechanical properties. Therefore, it suggested that these flowable composites can be used successfully in posterior restorations (6). Nanotechnology has provided the composite resins with smaller filler particle sizes in nano-scale dimension (3). These composites are suggested as an alternative to amalgam in the posterior teeth due to high strength, low shrinkage and acceptable aesthetics (7). Grandioflow and Grandio contain 80% w/w (65.6% volume) and87% w/w (71.4% volume) inorganic filler, respectively. Hence, both composites have high percentages of inorganic fillers that could affect final mechanical characteristic of the restorations. Polymerization shrinkage in flowable composite is one of the reasons that could create a gap between the composite material and tooth surface (8). This gap could cause penetration of bacteria, fluids, molecules and ionic particles to the interface of the tooth structure and restorative material. This process so called microleakage(9) is the main reason for excessive sensitivity, recurrent caries, pulp irritation and fractured restorations (10).

Despite improvements, no material has yet been able to create a full adaptation between restoration and cavosurafce margins (11, 12). The total-etch technique removes the smear layer and is used in a three-stage (4th generation) or a two-stage (5th generation) fashions (13). The most recent released product in adhesive materials is the seventh generation bonding agent which represents the latest simplification of adhesive system (14). The main advantage of this system is the elimination of the etching process and the use of primer and bonding at the same time. This helps to save time that was used for etch and rinse of teeth which eventually reduce the time of restoration procedure significantly. Shorter treatment time is highly valued in pediatric dentistry (15). For self-etch adhesives, surface demineralization of dentin and penetration of monomer into dentin happen at the same time (16). Hence, self-etch adhesives are suitable for pediatric dentistry because of short and simple restoration procedures they offer(17). Considering the superior characteristics of flowable compared to conventional composites, this study aimed to compare the microleakage level of nanohybrid flowable and conventional composites using total-etch and self-etch bonding agents

Materials and Methods

This study was approved by ethics committee of Qazvin University of Medical Sciences (IR.QUMS.REC.1394.64). For this experimental study, 48 extracted human primary molar teeth (D, E) due to orthodontic reasons or severe infection with at least one intact proximal surface were selected. After debridement of the teeth, they were stored in 0.5% Chloramine solution for seven days and then kept in distilled water at room temperature. Class II cavities were prepared on mesial surface of second primary molar or distal surface of first primary molar. Cavity preparation was performed to 4 mm Buccolingual width, 2 mm axial depth, gingival margin was placed in enamel and 1mm above the CEJ. Based on type of restorative composites and bonding agents, 4 goups were described as below:

1. GrandioFlow Composite (Voco GmbH, Cuxhaven, Germany)  Futurabond DC (Voco GmbH, Cuxhaven, Germany) as the seventh generation bonding agent
2. Grandio Composite (Voco GmbH, Cuxhaven, Germany) with Futurabond DC
3. GrandioFlow Composite Solobond M (Voco GmbH, Cuxhaven, Germany) as the fifth generation bonding agent
4. Grandio Composite with Solobond M

For Futurabond DC, the cavity was rinsed and dried after preparation with a faint air jet to prevent desiccation of the cavity surfaces. Futurabond Dc single dose was activated according to the manufacturer’s recommendation, and a layer of bonding was applied on the surface for 20 seconds, dried with an air jet for at least 5 seconds, and cured for 10 seconds (Woodpecker Guilin, Guangxi, China). For Solobond M, after cavity preparation and according to the manufacturer's instructions, cavities were etched using phosphoric acid 34/9% (Vococid, Voco GmbH, Cuxhaven Germany) for 20 seconds. Then the cavities were rinsed with water for 20 seconds, dried and a layer of bonding was applied on the surfaces, allowed to sit for 30 seconds, and cured for 20 seconds. After applying adhesive on the surface, cavities were restored with Grandioflow Composite as flowable nanohybrid composite and Grandio Composite as a conventional composite with a maximum thickness of 2 mm and curint time of 20 seconds. After completing the cavity restorations, fillings were polished with fine polishing bur (Teeskavan, Tehran, Iran). Teeth were stored in normal saline at room temperature for 24 hours. All teeth were immersed in water baths at 5 and 55°C for 1000 cycles, with a dwell time of 60 seconds in each bath and a transfer time of 15 seconds. After thermocycling, apical foramina of each tooth was sealed with wax, and the entire surface of tooth was covered with two coats of nail polish from apex up to 1 mm of restoration margin (Padina, N103, Iran). Teeth were immersed in 1 molar silver nitrate solution (Ranbaxy India. Ltd) for 24 hours. Later, teeth were mounted in transparent acrylic resin (Dentsply, Dentsply Detrey GmbH, England). Then the teeth were washed for 5 minutes with water and placed in a developing solution (Champion, X-ray Iran, Iran) under fluorescent light overnight. After dye exposure, samples were washed with purified water for 5 minutes. Restorations were sectioned mesiodistally using a diamond blade (0/005mm) and water as coolant. (Mecatome T 201 A, Pressi, France).  Sections from middle part of the restoration of each tooth were evaluated by stereomicroscope at 30X times magnification (N76097.98, MЂC-2, Russia) Microleakage evaluation and dye penetration for each sample was evaluated by a person who was blinded to study groups. Dye penetration was measured from margin of dental restoration up to the end of dye penetration at both side of sections. The greater penetration was considered as reference. Microleakage was recorded based on the following criteria (13):

Score 0: No dye penetration (Fig. 1):

 Score 1: Dye penetration along the occlusal or gingival wall up to one-third length of restoration. (Fig. 2):

Score 2: Dye penetration along the occlusal or gingival wall up to two-third but not less than one-third of the restoration length. (Fig. 3):

Score 3: Dye penetration along the whole length of the occlusal or gingival wall and along the axial wall. (Fig. 4):

Stereomicroscope images of microleakage scores of composite restorations (magnification30×)

Statistical analysis

The data collected was analyzed using SPSS version 19 software. Non-parametric Kruskal-Wallis test was used to compare microleakage in four groups. A pairwise comparison of microleakage was conducted using the Monte Carlo test. The level of significance was established as P ≤ 0.05 for all tests.

Results                 

Scores of microleakage at gingival margin of restorations in various groups are presented in Table I. The non-parametric Kruskal-Wallis test didn’t show significant difference among the groups (P=0.056; Table I). Pairwise comparison with the Monte Carlo test also did not show significant difference between any of the groups (P>0.05) (Table II).

Table I: Frequency distribution of microleakage at the gingival margins in the studied groups

Table II: Pairwise comparison between groups