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ORIGINAL ARTICLE
Year : 2019  |  Volume : 22  |  Issue : 5  |  Page : 633-641

Comparative evaluation of marginal adaptation and microleakage of low-shrinking composites after thermocycling and mechanical loading


Department of Restorative Dentistry, Faculty of Dentistry, Suleyman Demirel University, Isparta, Turkey

Date of Acceptance22-Jan-2019
Date of Web Publication15-May-2019

Correspondence Address:
Prof. R B Ermis
Department of Restorative Dentistry, Faculty of Dentistry, Suleyman Demirel University, Isparta
Turkey
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/njcp.njcp_567_18

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   Abstract 


Aims: The aim of this study was to evaluate and correlate marginal adaptation and microleakage of different low-shrinking composites. Materials and Methods: Standardized class V cavities (n = 20/group) with occlusal margin in enamel and gingival margin in dentin were restored with low-shrinking silorane-based (Filtek Silorane) or methacrylate-based (Clearfil Majesty Posterior, Grandio, Reflexions XLS, Tetric EvoCeram, Premise, Ceram-X Duo, Aelite LS Posterior) composites and a conventional composite (Filtek Z250). All teeth were stored in water (24 h), thermocycled (5000×), and mechanically loaded (60,000×). Marginal adaptation of epoxy resin replicas was analyzed using scanning electron microscope. Microleakage of the restoration was assessed by dye penetration on sectioned specimens under stereomicroscopy. Data were statistically analyzed with Mann–Whitney U test, with a significance of P < 0.05. Pearson's correlation test was used to evaluate the correlation between results of margin analysis and microleakage. Results: No statistical difference in marginal gap formation was determined between Filtek Silorane and Z250. The lowest microleakage score at dentin margins was recorded for Filtek Silorane, which was not significantly different from that of all other groups. No similar ranking between the results of microleakage at enamel and dentin margins was observed for the materials tested. Marginal adaptation was not correlated to microleakage, except for Filtek Silorane, Grandio, and Filtek Z250. Conclusions: Compared to the conventional composite Filtek Z250, Filtek Silorane demonstrated no improvements with reduced marginal gap formation. Low-shrinking properties of composites appear to have no ability of sealing restoration margins and preventing leakage.

Keywords: Dental leakages, low shrinkage, marginal adaptation, methacrylate-based, silorane resin


How to cite this article:
Hepdeniz O K, Ermis R B. Comparative evaluation of marginal adaptation and microleakage of low-shrinking composites after thermocycling and mechanical loading. Niger J Clin Pract 2019;22:633-41

How to cite this URL:
Hepdeniz O K, Ermis R B. Comparative evaluation of marginal adaptation and microleakage of low-shrinking composites after thermocycling and mechanical loading. Niger J Clin Pract [serial online] 2019 [cited 2019 May 26];22:633-41. Available from: http://www.njcponline.com/text.asp?2019/22/5/633/258281




   Introduction Top


Despite the improved esthetics and physical properties of composite resins, the marginal integrity of the restoration remains a challenge for dentistry.[1],[2] The shrinkage during polymerization process creates contraction stress at the vulnerable interface, which leads to gap formation.[3],[4],[5] The composite resins require reduced rate of polymerization shrinkage, increased bond strength to dental hard tissues, and ability to eliminate microleakage to achieve the properties necessary for long-term durability.[6]

To enhance the properties and the longevity of the material, one of the strategies was to reduce the total amount of monomer content through the addition of filler. As filler content increases, overall polymerization shrinkage decreases. This is because, shrinkage is confined to the resin monomer matrix.[6] Another common approach applied to decreasing shrinkage is the utilization of prepolymerized fillers that are manufactured by adding inorganic micro- or nanofillers at very high concentrations to a resin monomer.[7] The application of nanotechnology to dentistry has enabled the production of nanocomposites by adding greater amount of filler particles in the range of about 0.1–100 nm into the resin matrix.[8],[9] The use of nanofillers results in decrease in a wear rate, and increase in the mechanical, optical, and esthetic properties of the composite.[10],[11],[12]

The volumetric polymerization shrinkage of composite resins has been reported from 1.5% to 5.5%.[13] Because the latest attempts have been to produce a composite that will consistently shrink less than 2% by volume, methacrylate-based composites have been marketed and advertised by the respective manufacturers as low-shrinking composites if the shrinkage is at or below 2% by volume.[14],[15]

New composite resin formulations have been developed for the purpose of not only increasing the volume of inorganic particles but also modifying the chemical structure of certain monomers or replacing them.[4] A new monomer system is obtained from the reaction of siloxane and oxirane monomers. So-called siloranes replace the methacrylate monomers in the resin matrix of composite resins.[16],[17] The cationic ring opening mechanism of oxirane monomers during the polymerization process reduces the volumetric shrinkage of composite to less than 1%.[18]

Because of the low-shrinking nature of the materials, the silorane-based and methacrylate-based composites can be expected to produce less marginal failure. Therefore, we investigated the marginal adaptation and microleakage of class V cavities restored with a low-shrinking silorane-based or methacrylate-based composite and a conventional composite. The correlation between the marginal adaptation and microleakage test results was also analyzed. The hypotheses tested were that there is no difference in terms of (1) marginal gap formation and (2) microleakage of low-shrinking silorane-based or methacrylate-based composites, and (3) there is no correlation between microleakage and marginal adaptation.


   Materials and Methods Top


Following ethical approval (reference no: 117), 90 caries-free human third molars were obtained and stored in a 0.2% sodium azide solution at room temperature. Standard class V cavities (4 mm mesiodistal width, 3 mm occlusogingival width, 1.5 mm depth) were prepared on the buccal and lingual surfaces of each tooth. The gingival margins were located 1.0 mm below the cementoenamel junction. No beveling was applied. After each cavity preparation, a digital caliper (Mitutoyo Co, Tokyo, Japan) was used to check the accuracy of the dimensions.

The specimens were randomly assigned to nine groups (n = 10) and restored using a low-shrinking silorane-based composite (Filtek Silorane) with a two-step self-etch adhesive (Silorane System Adhesive) or one of the low-shrinking methacrylate-based composites (Clearfil Majesty Posterior, Grandio, Reflexions XLS, Tetric EvoCeram, Premise, Ceram-X Duo, Aelite LS Posterior) and a conventional composite (Filtek Z250) with a three-step etch-and-rinse adhesive (Adper Scotchbond Multipurpose) [Figure 1]. The classification, composition, manufacturers, and application procedures of composite and adhesive materials are described in [Table 1] and [Table 2]. The composites were placed in one increment and light cured for 40 s (600 mW/cm2, Demetron LC, Kerr, Orange, CA, USA). Final finishing and polishing of restorations were performed using fine grit diamond burs and flexible disks (Fini, Pentron, Wallingford, Germany).
Figure 1: Schematic explaining the study set-up. SSA=Silorane System Adhesive; SBMP=Adper Scotchbond Multipurpose

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Table 1: Materials used in this study. The data were provided by their respective manufacturers

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Table 2: List and application procedures of composite materials and their respective adhesives

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All teeth were stored in distilled water at 37°C for 24 h in incubator. The specimens were subjected to 5000 cycles in a thermocycling apparatus (custom-made, Nova, Konya, Turkey)[19] with two baths at 5 ± 2°C and 55 ± 2°C each with a dwell time of 30 s and a transfer time of 5 s between each bath. Mechanical loading process was performed using a chewing simulation device (custom-made, Selcuk University Research Laboratory Center, Konya, Turkey).[20] The testing was performed using 60,000 cycles with a force of 50 N at a frequency of 1.2 Hz.

For qualitative and quantitative evaluation of marginal quality, impressions of the restored teeth were taken using polyvinyl siloxane material and replicas were made with an epoxy resin. All cavity margins were analyzed by scanning electron microscope (SEM) with a high pressure at 10 kV and under 200× magnification (Vega II, LSU, Tescan, UK). Cavity margins were evaluated according to the following criteria:[21]

Continuous margin: The interface between the restorative material and tooth structure exhibits a smooth surface without any interruption in continuity; Marginal gap: The interface between the restorative material and tooth structure is separated by a gap; Artefact: The interface between the restorative material and tooth structure cannot be exactly assessed, e.g., due to overhanging excess material or errors in the replication procedure. For quantitative analysis, length of margins used for each criterion in relation to the total length of the cavity margins was calculated and expressed as percentage.

For microleakage evaluation with dye penetration method, the apex of each tooth was sealed with a light-cured composite resin and the surfaces were covered with two coats of nail varnish except 1 mm around the restoration margins. The specimens were immersed in a 0.5% basic fuchsin solution (Merck KGaA, Damstadt, Germany) for 24 h at 37°C in incubator. Thereafter, the samples were washed with tap water, air-dried, and embedded in an acrylic resin after nail varnish was removed. The acrylic blocks were cut bucco-lingually into three sections (mesial, middle, distal) with a water-cooled low-speed diamond disc, and two margins for both the enamel and dentin per section were recorded along the restorations to evaluate the dye penetration [Figure 2].
Figure 2: Example of tooth section, indicating class V restoration on buccal and lingual surfaces (arrows). En: Enamel; Dn: Dentin; DEJ: Dentin-enamel junction; Cr: Composite resin. Black straight line and white broken line indicate the entire length of enamel and dentin, respectively

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The sections were photographed with a digital camera (D-Lux 3, Leica, Wetzlar, Germany) and evaluated using stereomicroscope (S4E, Leica Microsystems, Wetzlar, Germany) at 40× magnification. Dye penetration degrees of the samples were scored as follows:[21] 0: No dye penetration; 1: Dye penetration up to the half of the wall; 2: Dye penetration more than half of the wall, 3: Dye penetration on the entire wall, including the axial wall.

Data were statistically analyzed using Kolmogorov–Smirnov test for normality of the distribution. Kruskal–Wallis and Mann–Whitney U tests were used to examine statistical differences in marginal quality and microleakage scores. Pearson's test was used to determine the presence of correlation between margin analysis and microleakage (P = 0.05).


   Results Top


Qualitative evaluation of margins with SEM demonstrated that continuous margins and marginal gaps were observed in all groups [Figure 3]. The results of quantitative evaluation of margins are presented per group in [Table 3]. Gap-free adaptation to cavity walls varied from 73.0% (Filtek Silorane) to 96.0% (Reflexions XLS). Kruskal–Wallis test revealed statistically significant differences among the groups for the criteria “continuous margin” (P < 0.05) and “marginal gap” (P < 0.05). The highest continuous margin percentages were obtained for Reflexions XLS and Clearfil Majesty Posterior, while Aelite LS Posterior, Tetric Evo Ceram, and Premise scored in between. No statistically significant differences were noted among these five materials (P > 0.05). The highest marginal gap percentages were measured for Ceram-X Duo and Filtek Silorane with no significant differences between these two groups (P > 0.05). Filtek Z250 exhibited significantly lower percentages of continuous margin than Reflexions XLS, Clearfil Majesty Posterior, Tetric Evo Ceram, and Aelite LS Posterior (P < 0.05).
Figure 3: Electron microscopy images, representing left (a and b) specimens showing the interfaces between material and tooth structure separated by a gap and right (c and d) specimens showing the interfaces with no interruption in continuity. En: Enamel; Cr: Composite resin; Dn: Dentin

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Table 3: Quantitative margin analysis scores for the different groups, determined as percentages of the length of margins examined (%) mean and standard deviation

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The distribution of microleakage scores of enamel and dentin margins of composite resins is detailed in [Table 4]. The detected percentages of no microleakage were in the range of 7.5% and 56.6% for enamel margins and 11.2% and 52.7% for dentin margins. Statistical analysis revealed significant differences among the groups for the microleakage scores at enamel margins (Kruskal–Wallis, P < 0.05). At the enamel, the lowest microleakage scores were found for Aelite LS Posterior, which exhibited no significant differences with Tetric Evo Ceram (P > 0.05) and Clearfil Majesty Posterior (P > 0.05), and which showed significantly lower microleakage than Filtek Silorane, Grandio, Reflexions XLS, Premise, Ceram-X Duo and Filtek Z250 (P < 0.05). Microleakage scores of Filtek Silorane were not significantly different from that of Reflexions XLS, Tetric Evo Ceram, Premise, Ceram-X Duo, and Clearfil Majesty Posterior (P > 0.05). Filtek Z250 showed a higher percentage of microleakage scores as compared to the other groups (P < 0.05), except Ceram-X Duo, while the difference was not significant only with Grandio (P > 0.05).
Table 4: Microleakage scores at enamel and dentin margins for the different groups, determined as percentages of the sections examined (%)

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No similar ranking between the results of microleakage at enamel and dentin margins could be observed for the materials investigated [Figure 4]. The lowest mean microleakage score at dentin margins was determined for Aelite LS Posterior, and Premise had the most microleakage scores at the dentin margins compared with all other groups. However, statistical analysis revealed no significant differences among the groups for the microleakage scores at dentin margins (Kruskal–Wallis P > 0.05).
Figure 4: Mean microleakage scores (score 0-3) for the materials investigated. The left part of the graphic presents scores at enamel margins; the right part shows scores at dentin margins

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The comparison of the microleakage scores between enamel and dentin margins within each restorative system exhibited significantly more leakage in dentin for Filtek Silorane Reflexions XLS, Tetric Evo Ceram, Premise, and Clearfil Majesty Posterior (P < 0.05). The other composite materials exhibited an equal marginal seal in dentin and in enamel. After pooling the data of all groups, the microleakage in dentin was found to be significantly higher compared with enamel.

[Table 5] presents Pearson's correlation coefficients (r) between microleakage and both continuous margin and marginal gap for the different groups. The correlation coefficient values ranged from −0.62 to 0.63 according to the obtained data. Considering all the materials used, Pearson's correlation test was indicated that there was a moderate positive significant correlation between the marginal gap and microleakage (r = 0.31, P < 0.05) and a moderate negative significant correlation between the continuous margin and microleakage (r = −0.32, P < 0.05). However, no significant correlation between the margin analysis and microleakage scores was determined in most composite materials, except Filtek Silorane, Grandio, and Filtek Z250.
Table 5: Correlation values between margin analysis and microleakage results within each material

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   Discussion Top


Many research approaches have been made to reduce polymerization shrinkage and the residual stress of dental composites and have led to the introduction of low-shrinking composites.[14],[15] Therefore, marginal adaptation and microleakage of adhesively bonded composite resin restorations were investigated in standardized class V cavities with the margins located in both dentin and enamel. One low-shrinking silorane-based (Filtek Silorane) and seven different low-shrinking methacrylate-based composites were tested in comparison to a conventional, relatively high-shrinking composite Filtek Z250. Because the low-shrinking composite Filtek Silorane can only be bonded its corresponding adhesive, the composite materials differed for the adhesive employed, but involved the same conventional composite Filtek Z250 as control.

The first hypothesis that there is no difference in marginal quality among the composite materials used was rejected, because significant differences for the marginal gap and continuous margin scores were observed (P < 0.05). The second hypothesis that there is no difference in microleakage at dentin and enamel margins of class V cavities among the composite materials used was partially rejected, as significant differences were observed among the materials for the microleakage scores at enamel margins (P < 0.05). The third hypothesis that there is no correlation between microleakage and marginal adaptation, was also partially rejected, because significant correlation between the marginal gap or continuous margin and microleakage was found for some of the materials used (P < 0.05).

Marginal analysis method assumes that if the stress generated by polymerization shrinkage or by thermomechanical loading exceeds the bond strength, a gap will be formed at the restoration margin.[5] The highest marginal gap percentages were measured for the silorane-based composite. Filtek Silorane required an adhesive specifically designed to bridge the hydrophilic tooth substrate with the hydrophobic silorane composite.[22] Despite its two-step application technique, its adhesion mechanism is similar to a one-step adhesive. Because the actual bond to the tooth surface results from the ultra-mild self-etching primer (pH: 2.7) only, the primer is light-cured separately before application of Silorane System Adhesive bond.[22] As ultra-mild one-step self-etch adhesives interact only superficially with the tooth tissues,[23] this may explain, at least to a certain extent, the highest gap formation observed with Filtek Silorane. On the other hand, the methacrylate-based composites were bonded with three-step etch-and-rinse adhesive (Adper Scotchbond Multipurpose). The two-step self-etch and three-step etch-and-rinse adhesives have been described for their superior bonding effectiveness compared to one-step self-etch adhesives.[3],[24]

Although both methacrylate-based composites and conventional control composite bonded with the same three-step etch-and-rinse adhesive, no significant difference was determined between Filtek Z250 and Filtek Silorane regarding the marginal gap formation. This suggests that the bond to the cavity margins was not sufficiently strong to resist the polymerization shrinkage stress induced by Filtek Z250. Filtek Z250 can be considered to be among the most shrinking composites (2.0–2.5%) with a relatively high E-modulus of 11 GPa.[25] The filler loading and E-modulus are the most influential properties of shrinkage stress for composite resins.[26] Combining both properties, the shrinkage stress created with the Filtek Z250 composite may be higher than the other methacrylate-based composites.[27]

Filtek Silorane did unexpectedly not exhibit decreased microleakage compared with any other methacrylate-based composites in this study. On the contrary, the lowest scores of microleakage at gingival dentin margins of class V cavities were recorded for the silorane-based composite resin, but the comparison was not statistically significant. Because of the reduced polymerization shrinkage of the silorane composite, the interface is exposed to significantly less stress,[28] so it is unlikely that shrinkage stress is the cause in this case. In a previous study, the significant decrease in microtensile bond strength of Filtek Silorane bonded with Silorane System Adhesive was obtained when bonding to cavity bottom dentin compared to flat dentin.[27] The authors attributed the decreased bond strength to the material properties rather than shrinkage stress accompanies the polymerization process by decreasing close adaptation of uncured stiff composite to the dentin surface in the narrow class I cavity design with high configuration factor (C-factor), known as the ratio of the bonded to the unbonded area.[27] The class V cavities have also unfavorable C-factors, resulting in high shrinkage stress scores within bonded composite resin material.[4] The results were possibly affected by the class V cavity design with increasing C-factor in this study. A similarly increased microleakage at enamel and dentin margins of the cavities with high C-factor was also reported by other authors regardless of the type of composite used.[15],[16]

Overall, marginal sealing ability of the materials was significantly better at the cavity margins located in enamel compared to dentin after thermocycling and mechanical loading. In several studies, it has been investigated marginal quality and microleakage of several restorative systems in class V cavities.[29],[30] Microleakage in dentin was also found to be significantly higher than enamel cavity segments for the two- or three-step etch-and-rinse adhesives compared with their one-step counterparts after thermomechanical loading.[17],[31] However, no similar ranking between the results of microleakage at enamel and dentin margins could be observed for the materials tested in this study. Interestingly, the silorane-based composite showed no higher dye penetration at enamel than most methacrylate-based composites tested in this study. Moreover, Filtek Silorane produced significantly lower values of dye penetration in enamel than Filtek Z250, as well as Grandio. These findings support the idea that the volumetric shrinkage is not the only factor to determine the marginal integrity.[32] Shrinkage stress is influenced by other material-related variables such as thermal expansion, modulus of elasticity and flowability and tooth-related variables such as the cavity configuration and size of the cavity.[6],[15],[26] This may also explain why no significant correlations were found between microleakage and marginal adaptation for some of the materials tested in this study, which is in agreement with a previous study.[33]

The microleakage at dentin and enamel margins for composite materials was higher in this study than in earlier reported studies,[30],[34],[35] which is probably due to the use of artificial aging methods. When thermocycling is applied to the specimens, repetitive contraction/expansion stresses are generated at the tooth–material interface resulting from the high expansion coefficient of composite resins.[3] Aging of restored cavities by occlusal loading in a chewing simulator may also affect adhesion to tooth structure because of fatigue. It has been reported that microleakage is less affected by water storage and thermal cycling[3],[36] than by mechanical loading.[37],[38] Although there is no consensus on a specific number of cycles for these methods, 5000 thermal cycles and 60,000 mechanical loading cycles were applied to the specimens to generate stress similar to the clinical situation in this study. It has been concluded that 10,000 thermocycles correspond approximately to 1 year of in vivo functioning, rendering 500 cycles, as proposed by the ISO standard, very minimal to mimic physiological aging in the oral cavity.[39]

For microleakage evaluation, dye penetration may provide an easy, fast, and commonly applied preclinical screening methods.[17] However, the interpretation of the minimal differences in the results is difficult, and it reduces the sensitivity because of a qualitative feature of the method. Different results seem to be affected by many factors in different studies, as well.[40] SEM is another widely used method for evaluating marginal adaptation. Direct observation by SEM requires an effective dehydration and drying procedure to avoid artefacts.[41] The indirect method using an epoxy resin model is another way to avoid artificial gap formation.[41],[42] The use of replicas was selected to avoid artefacts from SEM sample preparation procedures in this study. Marginal areas observed with the artefact criteria were assigned to a low percentage for four groups only (<1%).

The sealing ability of these restorative materials should also be examined by advanced technologies, including noninvasive cross-sectional imaging technique of optical coherence tomography and three-dimensional technique of microcomputed tomography. Hence, further long-term in vitro and in vivo studies are required to demonstrate the marginal sealing ability of different composite materials.

The following conclusions were drawn based on this study. Compared to methacrylate-based composites, low-shrinking silorane-based composite demonstrated no improvements with reduced marginal gap formation. Compared to low-shrinkage methacrylate-based composites, the silorane-based composite did not show significant microleakage reduction. Low-shrinking properties of different types of composite materials do appear incapable of sealing restoration margins and thus preventing marginal integrity and leakage. Gap-free and continuous margin formation is not easily related to microleakage because not all the materials tested demonstrated a significant correlation. Marginal sealing ability is material-dependent and seems to be affected by the characteristics of materials and study design.

Financial support and sponsorship

This study was supported by the Suleyman Demirel University Scientific Research Projects Foundation (1855-D-09).

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5]



 

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