Introduction

Nowadays, the use of various types of composites is preferred by both dentists and patients because of their beautiful appearance and conservative technique. Although mechanical and physical properties of such composites have been improved in recent years, the contraction of polymerization still is their main drawback (1).

Stress-induced "polymerization shrinkage" can lead to detachment of the dentin-composite junction and consequently marginal leakage will occur (2). Micro-leakage is a dynamic phenomenon which is mainly caused by the penetration of liquids, bacteria, and ionic compounds into the prepared cavity wall and restoration materials. This phenomenon has been regarded as the main cause of composite restoration failures, secondary caries, hypersensitivity, and pulp inflammations (3, 4) .In this regard, in recent years much attention has been paid to the use of adhesive systems in order to promote the dentine-composite junction, minimizing micro-leakage, and preparing dental cavities with the minimum amount of tissue removal (5).

Adhesive systems have been categorized into two main categories based on their methods of application and their connection mechanisms; self-etching adhesive systems and etch & rinse adhesive systems
 (1,2,6). In the former category, the dentin etching process does not carry out and the acidic monomers presented in the adhesive system etches the dentin and enamel surfaces and thereby facilitates the penetration of resin in the demineralized dentin (5). Accordingly, the self-etching adhesive systems are preferred because the rinsing and drying steps are deleted, the required time would be reduced, and also there are lesser opportunities for human errors to occur (7-9).  

In recent years laser systems have found many applications in preparing and processing of surfaces of organic and inorganic materials (9, 10).

The first use of such systems in dentistry field went back to the beginning of 1960 decade and Laser application in dentistry has been increasingly increased during the recent two decades. Given the vast range of laser systems' capabilities, it can be utilized for various purposes in the field of dentistry. The followings are some dentistry-related processes which can be performed using laser systems; the preparation of dental cavities, caries removal, removal of previous restoration, etching of enamel and dentin surfaces, treatment of hypersensitivity, caries prevention, and bleaching (10-13).

Having a wide range of frequency, capability in shaping material surfaces, high energy density, and capability of being concentrated on a point are some of the advantages that laser systems benefit from, and why such systems have found many applications in various area (9).

The most conventionally used laser systems in dentistry include CO2 laser, Nd:YAG (Neodymium: Yttrium-Aluminum-Garnet), Erbium  family lasers
(Er: YAG and Er, Cr: YSGG), and Diode lasers (14).  Diode laser system is a common one used prevalently in the dentistry field. It is utilized a semi-conductor media to produce a laser beam. Gallium-aluminum-arsenide (GaAlAs) is the best known type of semiconductor laser with a wavelength between 800 and 980 nm (15).

In recent years, dental researchers have made many attempts to investigate the effects of laser irradiation on the adhesive systems which have been located on the dentin surface before being polymerized by light. As far as we know, the research conducted by Goncalve et al has been the first one in which the effect of laser beams on adhesive systems are investigated (16, 17). Moreover, the studies to assess the effects of such systems on the micro-leakage rate are rare. Kawaguchi et al is one the few studies which have been conducted in this area, which reported no effect of laser systems on micro-leakage. Therefore, there is a need to conduct more studies in this area to find out how the laser systems can affect micro-leakage of composite restoration (18).

Accordingly, the present study was conducted to investigate the effect of Diode laser (940 nm) irradiation on micro-leakage of class V composite restoration before and after applying the bonding agent.  

Materials and methods

In this experimental study, 60 human premolar teeth which were free of cavities, previous restorations, fractures, and wearing were selected to be investigated (19). Several steps were conducted to prepare the teeth for the study; 1. Soft tissues remaining on the teeth surface were removed by a hand scalar; 2. The teeth were thoroughly washed and cleaned by pumice; 3. They were examined under a light microscope for assuring that they met the inclusion criteria of the study; 4. They were disinfected by submerging under 0.2 percent thymol solution until 24 hours before the study; 5. After which they were placed in distilled water at room temperature.

Teeth preparation for investigating the
micro-leakage

On the buccal and lingual surfaces of each tooth a standard U-shape cavity with dimensions of 1.5 mm (buccolingual) * 3 mm (mesiodistal) * 2 mm (occlusoginigval) was created utilizing a Diamond Cylindrical Grinding device (Tizkavan Tehran, Iran) and a high speed handpiece (BienAir, Switzerland). The cavity was created so that its occlusal margin was 1 mm above CEJ and its gingival margin was 1 mm lower than CEJ.

Groups of the study

In the next, the teeth were randomly categorized into four equal groups (A, B, C, D) according to the Simple randomization method (Roll of a die);

•        Group A (control group): primer + bonding agent + composite

In accordance with the manufacturer (Kuraray Dental, Japan), the primer was implemented for 20 seconds. Then, air drying was done and the bonding agent (Kuraray Dental Clearfil SE Primer, Japan) was located on the cavity surface of the teeth. Next, the teeth were cured using Demetron A.2 (Kerr, Germany) for ten seconds at 1100 mW/cm2. Samples were covered by resin composite (FiltekTMZ250, 3M ESPE, USA) with A2 color, height of 4 mm, and 2 mm thickness. Then, samples were cured for 40 seconds. At the end, the overall treatment was cured for 40 seconds again. The composite and bonding system (sixth generation, Clearfil SE bond, Kuraray, Japan) were the same for all groups.

•        Group B: primer + Diode laser irradiation + bonding system + composite 

All steps were the same as those of Group A, except that the diode laser irradiation was applied after the implementation of primer. The characteristics of the laser beam are presented in Table 1. Moreover, the laser beam was irradiated from a distance of 5 mm, by a sweeping motion, and for 30 seconds.

•        Group C: primer + bonding system + Diode laser irradiation + composite

All the steps performed for treating the teeth of this group were the same as those used for the teeth of Group B, except that the diode laser irradiation was implemented after adding the bonding agent and before using the light polymerization. The characteristics of the laser beam were the same as those of group B (Table 1).

•        Group D: primer + Diode laser irradiation + bonding system + Diode laser irradiation + composite

For treating the teeth of this group, the laser irradiation was used both before and after the implementation of bonding agent. Other procedures were the same as those of other groups.

Table 1. the characteristics of the laser beam

Investigation of micro-leakage

The following steps were conducted in order to investigate the microleakage at each group; 1. Samples were submerged in water for 24 hours at 37 C; 2. The teeth were polished using special discs
(Soflex, 3M ESPE, USA) and water; 3. Samples were exposed to 5-55 C heat cycles for 1000 times in a thermal bath (Nemo Mashhad, Iran) with a dwell time of 30 seconds and a transfer time of 5 seconds; 4. All teeth surfaces as well as mesial and distal surfaces were covered by two layers of nail varnish; 5. The samples were submerged in a basic fuchsine dye solution at room temperature for 24 hours; 6. After this period of time, samples were washed by water stream for 5 minutes; 7. To fuchsine dye to be stabilized, the samples were dried by placing at room temperature for 24 hours; 8. In the next step, the teeth were cut into two parts by a cutting machine (Nemo Mashhad, Iran); 9. Finally, the teeth were assessed by a Stereo microscope (Olympus SZX16, Japan) at a magnification of 40× to rank their status in terms of microleakage based on the following criteria; (Fig 1,2).

0-      There was no penetrated dye in the tooth

1-      The penetration of dye in the gingival or occlusal directions was less than 1/3 cavity wall.

2-      The penetration of dye in the gingival or occlusal directions was between 1/3-2/3 cavity wall.

3-      The penetration of dye in the gingival or occlusal directions were higher 2/3 but not reached the axial surface.

4-      The penetration of dye was reached the axial surface.

Statistical analysis

In the present study, non-parametric Kruskal-wallis and Mann-Whitney tests were employed to compare the groups.

Results

For assessing the difference among the groups in terms of their microleakage level from the gingival margin (Table 2), Kruskal-wallis nonparametric test was employed, which showed no significant difference among the groups (p-value=0.116), so no further analysis was conducted (Table 3).

The same test was used to assess the difference among groups in terms of their microleakage level from occlusal margin, the results indicated a significant difference (p=0.015), so the analysis was followed by Mann-Whitney test to compare the groups in a pairwise manner. Table 4 represents the results of Mann-Whitney test, performed to compare the groups with each other in terms of their microleakage from occlusal margin status. As evident in this table, there was a significant difference between group A (control group) and group B (in which the laser beam was irradiated after the primer) (p=0.029). Moreover, the difference between group A and group D was also of significant importance (p=0.041). Furthermore, there was a significant difference between group B (in which the laser beam was irradiated after the primer) and group C (in which the laser beam was irradiated after the implementation of bonding agent) (p=0.029). The difference between group C and group D was also significant (p=0.037).

Table 2. Frequency distributions of microleakage scores (percentages) on occlusal and gingival margins

among the different groups tested.

Table 3.The results of Kruskal-Wallis test for comparing the amount of gingival and occlusal marginal microleakage

Table 4. The results of Mann-Whitney tests for comparing the groups with each other