INTRODUCTION
Due to their optimal characteristics and properties, resin composites are the materials most frequently used in the daily practice of a large number of dental specialists, even though they are provided with volumetric contraction as a result of the conversion of resin monomers from the organic matrix and their conversion in polymer mesh, all of which causes a decrease in resin volume, multiple forces which produce gaps and microgaps, and the tooth′s subsequent marginal leakage in relation to dental tissue.1 All these effects allow a permanent entrance of bacteria and oral fluids, and therefore sensitivity, secondary cavities, and irreversible pulp damage.2
This microleakage, occurring at the tooth-restoration interface in the form of gaps created during the application of composite or while it is working, is the main cause of failure of adhesive restorations, affecting retention and marginal adaptation to the subsequent elements involved in this type of failure.3 In an attempt to reduce this phenomenon, the use of cavity ionomeric bases and dentin bonding resinous agents have become popular, and their proper application seeks to minimize the adverse effects of polymerization by contraction and the consequent decrease or absence of marginal leakage.4
However, marginal leakage is linked to the longevity of restorations, and it may start right at the moment of application of the material or during its increase while the restoration lasts. There is no clear and reliable clinical evidence of restorative materials with sufficient properties to restore anatomy and dental functions while remaining intact for long periods of time and maintaining integrity and tooth structure, producing deterioration of the materials around the interface proportionally to the years of function.5 All this happens in spite of the development of dental biomaterials, making it impossible to achieve total sealing between tooth and composite in order to prevent microleakage,6 as the adhesive resin materials are not the only ones that determine success or failure, but also even factors connected to the structure and tissue where the procedures are being implemented -in addition to the clinician′s actual intervention-. Based on these premises, this study seeks to determine changes in terms of the degree of microleakage occurring at the tooth/restoration interface at different periods following preparation.
MATERIALS AND METHODS
The sample of this study included 60 healthy third molars with closed apex, which were extracted due to therapeutic indications and kept in a moist environment. The samples were subjected to class V cavities in buccal and palatal/lingual, and a metallic matrix of 3 mm was used in side and depth. In addition, diamond pyriform burs (MDT, ISO 237-010M) were used at high speed and under refrigeration, changing them every 5 preparations.7
Once the cavities were prepared, 5.25% sodium hypochlorite was applied on all the cavities for 1 minute, both in enamel and dentin, as well as 35% "Vococid Gel" (Voco) phosphoric acid for 15 s in enamel and 5 s in dentin; then the samples were rinsed for 20 s with copious water and dried for 5 s with swabs of absorbent paper. Next, the "Admira Bond" (Voco) adhesive system was applied in two layers, light curing it with a halogen light (Litex 680. USA) for 20 s at a cone distance of 5 mm by means of a wooden box that was devised for this purpose. The "Admira" (Voco) composite resin was applied in increments of 3 mm and in two layers of 1.5 mm each, by means of the incremental technique, light curing each layer with a halogen light (Litex 680.USA) at a distance of 5 mm for 40 s and using the same aforementioned standardization distance system.
Once the restorations were completed, the samples were polished 24 hours later with soflex discs (3M, ESPE) and root apexes were sealed with the "Admira Flow" (Voco) flowable resin. A layer of nail polish was applied on the entire surface of the teeth with a margin of 2 mm around the restorations in order to completely seal the tooth surfaces; this procedure was performed in all the samples.
In this process, the samples were randomly divided into three groups (n: 20). Each group was subjected to a thermocyling process of 5,400, 8,100 and 10,800 cycles at temperatures ranging from 5, 37, and 55° C in a thermocyling machine (Teratonic, Ecuador). After the established times, a new layer of nail polish was applied on the entire surface and the samples were immersed in methylene blue colorant for 24 hours. Then they were cut into two fragments in an occlusal-apical direction and evaluated by means of a stereoscopic microscope that allowed measuring the penetration of colorant in the occlusal and cervical surfaces. The obtained data were entered in Excel spreadsheets and later analyzed using the SPSS software.
RESULTS
The data obtained from the three groups were tabulated and totaled as shown in Table 1. They were analyzed by means of Student′s t-test yielding p & 0.05 which indicates that there is no tendency to increase microleakage values in relation to thermocycling times (Figure 1). To verify these findings, the ANOVA test was run, showing no significant differences in terms of microleakage according to measurements in the different groups, with p & 0.05 (Table 2). These results were confirmed by a Tukey test as a supplementary analysis, showing p & 0.05 which confirms the absence of significant differences among the groups.
DISCUSSION
The development of restorations with the capacity of bonding to tooth structure in an effective, stable and durable manner has been a major achievement in adhesive dentistry. The resistance of these materials, as well as their stability in the presence of masticatory forces and temperature changes, are characteristics that guarantee the permanence of these materials in the oral cavity.8
Previous studies have shown that thermocycling is the most convenient way of measuring microleakage, simulating the passage of time and the aging of materials in vitro.9)(10 However, maintaining a constant temperature of containers hardly reproduces oral conditions in a reliable manner; nevertheless, thermal cyclers have been accepted as machines with the ability to simulate the desired natural aging, and that is why this study used a thermocycling machine with containers graduated at 5, 37, and 55° C, in an attempt to reproduce the passage of time in a safe and reliable manner.
The role of sodium hypochlorite as deproteinizing agent is extremely important in restorative protocols; depending on its concentration, it contributes to the removal of the organic tissue′s smear layer, being recommended in dental operation procedures at a concentration of 5.25%, which allows more antibacterial capacity and greater effectiveness as organic tissue solvent.11 This validates the use of such concentration in this study, even though it was not evaluated by comparing it to other agents. Several authors claim that sodium hypochlorite affects adhesion in dentin; however, this hypothesis was not verified in this study, and therefore future studies are needed to compare adhesion and microleakage using different agents.
Methylene blue has a pH of 5.5, which is similar to the acid environment of bacterial metabolites, and the size of its particles is similar to the size of bacterial particles and their metabolic products;12 this is why it is convenient to evaluate microleakage using this method of contrast, along with apical sealing and tooth isolation with nail polish directly applied on the restoration.
The restorative protocol was closely followed, respecting the light curing times suggested by the manufacturer, using the incremental technique of composite until final polishing. Similarly, as any alteration of the protocols established by the manufacturer may modify the results in terms of the properties of restorative materials (especially the adhesive ones), the diamond burs used during cavity preparation were changed every 5 restorations based on the apparent loss of granulation, which results in lower effectiveness.7 The application of phosphoric acid and adhesive system in their correct times is important, and following the protocol in terms of quantities and times is definitely a determinant factor of success or failure in adhesive processes; however, we consider that a trained and qualified operator definitely makes the difference, as confirmed by the reported results of our study.
The analyses showed that thermocycling affects the marginal sealing of these restorations proportionally to the cycling times to which they were subjected; this happened in a similar way in both occlusal and cervical surfaces in the samples subjected to short periods of time, with values tending to increase in cervical proportionally to the aging time, in a stable and homogeneous way in both walls. This disagrees with other authors who claim that the cervical wall is affected to a greater extent in comparison with the occlusal wall.13 In our study, microleakage did not show significant differences between the walls or among the groups under evaluation. These results agree with the analysis by other authors as well as with studies on the quality and longevity of restorations located in the posterior sector,14 which demonstrate that class II restorations have an average life span of 5 years and class I restorations an average of 7 years, thanks to the huge improvements in terms of composition and characteristics of resinous materials and adhesive systems, which make them a reliable alternative for anterior and posterior sectors of the oral cavity-as observed in this study-, thus ensuring a long life to the restoration, provided that the operation instructions and conditions were appropriately met and fully controlled.
These findings raise the future need for research comparing the degree of microleakage with different types of disinfection agents, which could appear as beneficial elements influencing the durability of restorations especially because of their remodeling action and to a certain extent as modifiers of dental elements and the residue left by the instruments used in the surgical process. In addition to hypochlorite, it is recommended to experiment with chlorhexidine, saline solution, and others substances in order to make comparisons in both enamel and dentin, because we are aware that time is extremely important, and studies should be performed to verify the longevity presented by manufacturers (from 5 to 7 years), verifying the possibility of any alterations during these times.
The implementation of a rigorous protocol is essential, especially when dealing with adhesive restorative procedures; it is also important to use properly controlled and maintained light-curing devices, as well as materials of optimal quality and trained operators who are aware of the importance of their work.
CONCLUSION
In comparing the degree of microleakage in the three periods evaluated in this study, dimensional stability was observed in terms of the tested restorative material and the tooth walls where it was applied, regardless of the simulated months of execution. Closely following an established protocol, as well as the pre- and post-operative precautions suggested by the manufacturer, ensure the restorative success of the resinous material.