Effect of Bonding Agent on Retention of Different Sealants: An in Vitro Study

INTRODUCTION

Pediatric dentistry guidelines recommend sealing the primary and permanent molars in children and adolescents [1]. The clinical success of fissure sealants (FSs) is highly related to their appropriate application [2]. A dry enamel surface is mandatory to achieve good adhesion [3].

However, the success of sealants is based on some characteristic features, and some of these characteristics include the prevention of ingression of oral fluids and bacteria between the sealant and tooth surface [4].

Few clinical and in-vitro studies advised using adhesive bonding agents under the sealant for improving sealant retention and decreasing microleakage [5, 6, 7, 8, 9, 10]. In contrast, other studies reported that a bonding agent applied under the sealant does not improve its retention and does not decrease microleakage [11, 12, 13, 14, 15, 16].

Most of microleakage tests methods require the use of a tracer or a dye and cutting the tooth into a series of sections to visualize the extent of staining along the tooth-restoration interface with scanning electron microscopy. The depth of dye penetration along the margin can be measured or graded with a scoring system [17, 18]. A shortcoming of these tests is that they provide a two-dimensional (2D) and semiquantitative evaluation of leakage because interfacial staining is visualized on a limited number of tooth slabs, and some tooth structure is inevitably lost with sectioning [19]. Furthermore, tooth sectioning is a time-consuming and destructive procedure that prevents further testing of the specimen.

Over the last decade, the use of micro-computed tomography (micro-CT) has had considerable development in dental research [20]. Micro-CT is a non-destructive method that, starting from a series of 2D images, produces a three-dimensional (3D) reconstruction of the observed specimen [21]. Lately, the technique has been proposed for the evaluation of marginal leakage in pit and fissure sealing [22, 23]. However, micro-CT has been validated in the assessment of marginal leakage at the interface between enamel and FS [23].

At present, there is no standardized method for the in-vitro evaluation of retention of FSs due to different pits and fissures anatomy [24]. Also, there are no studies till date is conclusive regarding the use of bonding agent with Co-curing or Staged-curing when compared to No-bonding in reducing the microleakage and improving the longterm clinical retention of resin-based sealants (RBS).

This research carried out an investigation regarding the effect of bonding agents with Co-curing or Staged-curing when compared to No-bonding on the retention of different RBS in permanent teeth.

The research null hypothesis was: There would be no significant difference in sealants retention with or without bonding.

MATERIALS AND METHOD

Ethical approval was obtained from the Research Ethical Committee at the Faculty of Dentistry, King Abdulaziz University (KAU), Jeddah, Saudi Arabia (Approval no. 089-16).

Laboratory Procedure

For SBS test each sample was held parallel to Universal Instron Testing Machine “INSTRON 5944 2KN, MA, USA” at the shearing rod. The crosshead speed of this testing machine was 0.5 mm/ minute and results obtained in Megapascals (MPa). The SBS values were measured automatically by using the computer system of the Instron machine. For microleakage test, all teeth were thermocycled using thermocycling machine (SD Mechatronic GmbH comp., Germany) for 1000 cycles between 5-55°C in a water bath with the dwell time of 10 seconds. Teeth apices were sealed with sticky wax to prevent dye penetration. Teeth surfaces were coated with three layers of nail varnish except for the surface of restorations and the surrounding 1mm. Teeth were immersed in 50% silver nitrate for 4 hours at room temperature [21]. All specimens were exposed to the fluorescent light to stabilize the stain. Then they were removed from the dye solution and washed under running water for half an hour. To obtain the X-ray images, each specimen was settled with sticky wax in the specimen holder of the micro-CT system (Model 1172 “Skyscan Kontich, Belgium). To scan all specimens, a setting of 1mm aluminium filter and 100 kV⁄98mA X-ray source was used. At 180 degrees, each specimen was rotated with a rotation step of 0.40 degrees. The gain was set at 1.0. and the exposure time was 3.7 seconds. The magnification was set at 20, which provided a pixel size of 13.4 lm. All the projected X-ray images had the cone-beam reconstruction using NRecon version 1.6 software. By the use of CT-Analyser V.1.11, axial images were obtained with the cross-sections perpendicular to the bucco-lingual direction of the cavity. To obtain raw data, cross-sections of specimens were collected and converted into 16-bit-mapped image files revealing 2-D images with a resolution of 512x512 pixels. Each sample was randomly given a digital code. The list was kept in a sealed envelope, which ensured that the evaluation would be carried out blindly by one examiner. The examiner was faculty from the Department of Pediatric Dentistry, KAU. The intra-examiner reliability was obtained using SPSS version 21.0 by computing weighted kappa. The kappa value was 0.74 which was in an acceptable range that indicated an acceptable level of consistency. The examiner scored the degree of dye penetration on an ordinal scale ranging from 0 to 3. The examiner had given only the worst score for each sample from both cross and axial sections. The following scoring criteria have been used according to Manhart et al [25]: 0 = no dye penetration (Figure 1), 1 = dye penetration to half of the interface length (Figure 2), 2 = dye penetration beyond half of the interface length (Figure 3) and 3 = dye penetration reaches the base, all around the sealant (Figure 4).

Figure 1. Cross and axial sections showing no dye penetration-Score 0

Figure 2. Cross and axial sections showing dye penetration to half of interface length-Score 1

Figure 3. Cross and axial sections showing dye penetration beyond half of the interface length-Score 2

Figure 4. Cross and axial sections showing dye penetration all around the sealant-Score 3

RESULTS

Frequencies of Microleakage among Study Groups

The outcomes for the microleakage for the scores have shown that most of the Seal-it (No-bonding), Seal-it (Staged-curing) and Seal-it (Co-curing) revealed dye penetration beyond half of the interface. Similarly, the outcomes for Helioseal-F (No-bonding), Helioseal-F (Co-curing), Clinpro (No-bonding) and Clinpro (Co-curing) have shown that most of the teeth revealed dye penetration beyond half of the interface. On the other hand, microleakage outcomes for Helioseal-F (Staged-curing) and Clinpro (Staged-curing) revealed that most of the teeth showed no dye penetration or dye penetration to half of the interface length (Figure 5).

Figure 5. Percentages of microleakage scores among study groups

DISCUSSION

This study compared three different RBS materials on pretreated enamel either with No-bonding, Staged-curing or Co-curing in permanent teeth by measuring the SBS and microleakage. Based on the results of this study, the null hypothesis was rejected because there was a significant difference in sealant retention with and without bonding.

According to our knowledge, this study was the first to assess the SBS of “Seal-it” material.

Bond strength was considered to be tested as it can reflect the retention and longevity of these sealants [26]. Evaluating the bond strength utilizing the SBS test has advantages such as the balanced distribution of stress, evaluation of smaller surfaces, reducing the effect of enamel defects and recognition of even small differences in bond strength. The reliability of this method was previously confirmed by the work of Placido et al [27].

Etemadi et al [28] assessed the reproducibility of a previously established cavity preparation design which should be manually reproduced by specialists and found a great difference in the preparations made by different individuals. In our study, the buccal surfaces received standardized cavities done with the milling machine with standardized depth by using machine digital measurement and standardized width by using bur size without interference by the operator to avoid differences in the preparations. Also, we used buccal surface modelling instead of fissure modelling to standardize our study and to avoid differences in pits and fissures anatomy that could lead to bias of results.

In the current study, we applied the manufacturer’s instructions during etching, bonding, sealant application and curing except in subgroups of Co-curing. We studied Co-curing procedure to evaluate the effectiveness in the improvement of sealant retention and to shorten the working time of placing both a bonding agent and sealant in permanent teeth, especially in uncooperative children. In the literature, the idea of using adhesives with sealant on permanent teeth with one curing procedure is not yet firmly studied and manufacturers may take this concept in consideration for their products and work on it.

In this study, to standardize our procedure, we used one type of fifth-generation bonding agent to evaluate its effect on the retention of three different sealant materials since using more than one type of a bonding agent may lead to bias in the concluded results. Among all generations of adhesive bonding, the fifth-generation is considered the best. Furthermore, all samples were thermocycled then stained under the same conditions. Scanning was done by one experienced technician and microleakage evaluation was done by one examiner blindly to avoid bias and methodological errors.

The three sealant materials were selected based on their different compositions. Clinpro is an unfilled RBS, Helioseal-F is a fluoride releasing, filled RBS and Seal-it is filled RBS and relatively new in the market.

In the current study, scoring assessment was used because leakage assessment by the use of computer software is dependent upon the density of the sealant and the bonding agent, as scattering of X-rays from the tooth surface creates various degrees of ‘noise’ in the background of the scan and upon resolution which is determined by the distance of the tooth to the X-ray source. For example, a low-density dental material may be difficult to distinguish from the ‘background noise’ and also, perhaps, from the enamel. In our study, we used an assessment of multiple surface scoring methods because assessing a single section of the tooth is not representative because dye penetration varies from one area to another [29]. We used 3D to evaluate microleakage through all samples, this agrees with Raskin et al [19] who recommended the utilization of three sections for each restoration to accurately evaluate leakage. Also, Gale and Darvell [30] stated that 3D techniques revealed markedly greater microleakage than 2D assessments.

In contrast to our findings, Boksman et al [31] found that the use of bonding agent before sealant application did not increase the retention. The results of this study showed the highest results of shear strength were detected in the Staged-curing subgroups, although the difference in SBS values was insignificant only between the Stagedcuring and No-Bonding with Helioseal-F groups. This could be explained by placing the intermediate bonding layer between etched enamel and sealant with significant retention that created strong micromechanical interlocking. In addition, the bonding layer may be affected by higher fluoride content of Helioseal-F that lead to no significant difference between No-Bonding and Staged-curing. Another possible explanation is use of different generations of bonding agents.

On the other hand, our findings agreed with research conducted by Feigal et al [8] on the effect of bonding agent on sealant retention, they showed that the bonding agent was effective in sealant retention. This could be explained by the created strong micromechanical interlocking formed by the easy flow of the bonding agent.

Pushpalatha et al [32] found out that Clinpro (unfilled sealant) showed better SBS in comparison to Helioseal-F (filled sealant). This agrees with outcomes of this study, as we found Clinpro showed the highest retention with all pretreatment procedures except Co-curing and this could be explained by the low viscosity of the unfilled Clinpro sealant. On the other hand, Co-curing may affect the flow of Clinpro causing lower retention than the other two pretreatment methods.

Additionally, the results from the comparative study of Asselin et al [6] showed that the bond strength of bonding and self-etching adhesive groups were higher than the group without bonding. In another study by Ahuja and Ahuja [5] enamel SBS was evaluated through these groups: Single bond, Clear fill protect, Clear fill SE bond, Admira bond and no bond application as the control group. They concluded that Single bond has the highest SBS. This is also in agreement with our study.

For all SBS values, the No-Bonding subgroups revealed a significant difference of retention. Higher retention was recorded for Clinpro followed by Helioseal-F then Seal-it. This agrees with the study of Al-Sarheed et al [33] who find out that Visio-Seal had significantly lower bond strength than all 3 other materials using the etchant system and this could be explained by the viscosity factor, in which porosities of the etched surface were filled more with sealant material which has lower viscosity.

A study on retention of FS with or without bonding agents by Pinar et al [15] concluded that the use of bonding agents as an intermediary layer between enamel and sealant did not affect sealant success during 24 months. Also, Mascarenhas et al [34] concluded that the use of bonding material with sealants does not produce any significant outcomes. However, these results disagree with this study and the possible explanation may due to using different types of sealant materials and bonding agents.

Based on the outcomes of this study, a significantly lower rank of microleakage was with Staged-curing subgroups in both Clinpro and Helioseal-F, while Staged-curing with Seal-it revealed insignificant the lowest rank of microleakage than all other pretreatment procedures. This agrees with Askarizadeh et al [35] who found that the reduction of microleakage was due to the placement of sealant with bonding. In addition, Tirali et al [10] concluded that pre-treatment adhesive procedures showed lower microleakage than acid etch thus justifying our study results. This may be due to the intermediate bonding layer that created strong micromechanical interlocking between the etched enamel and the sealant which decreased leakage between the tooth structure and the FS.

Furthermore, Cehreli and Gungor36 used the image analysis toolkit for evaluating the microleakage quantitatively. Regardless of the storage term, the use of etch and rinse adhesives resulted in significantly less microleakage compared to that achieved with self-etching adhesives or acid etching alone. The outcomes obtained from the present research study also demonstrated that the use of adhesive and bonding material decreased the rate of microleakage to some extent. However, they concluded that after four years, sealants placed without a prior bonding agent revealed an enormous amount of leakage and this agrees with our study.

In our study, Co-curing showed the highest microleakage between pretreatment procedure groups. Birlbauer et al [37] used materials of three different formulations of an experimental fissure primer (EFP) that was applied without using phosphoric acid etching (EFP-1/EFP-2/EFP-3) and included one control group with sealant application after 30 seconds of acid etching. They concluded that microleakage was significantly lower in the control group than in EFPs groups. This could be due to that Co-curing and EFPs did not form a strong bond between the tooth structure and the sealant material as the conventional etching or the addition of a bonding agent.

This study has some limitations including the relatively long time, which is needed to collect sound permanent teeth. Also, interpretation of these results should consider the limitations of this in-vitro study when compared to clinical trials. In addition, the use of buccal surface modelling instead of fissure modelling could have affected the results. There are clear differences between fissure surface and flat ground buccal enamel surface, such as the presence of aprismatic enamel in fissures and differences in configuration factors.

CONCLUSIONS

The use of a bonding agent as Staged-curing before placement of a RBS in permanent teeth was more effective in increasing SBS and decreasing microleakage than No-bonding and Co-curing.

ACKNOWLEDGEMENTS

This project was funded by the Deanship of Scientific Research (DSR) at King Abdulaziz University, Jeddah, Kingdom of Saudi Arabia under grant no. G-25-165-38. The authors, therefore acknowledge with thanks DSR for technical and financial support.