| Abstract|| |
Aim: The aim of this study was to compare the microleakage in Class II box preparations with the gingival margin above and below the cemento-enamel junction (CEJ) restored with Silorane composite and methacrylate composite using two different layering techniques.
Materials and Methods: Standardized box preparations (mesial box 1 mm above the CEJ and distal box 1 mm below the CEJ) were prepared in 60 upper premolars. The teeth were randomly divided into four groups containing 15 samples each; Group I: Restored with a Silorane composite using an oblique layering technique, Group II: Restored with Silorane composite using a vertical layering technique, Group III: Restored with methacrylate composite using the oblique layering technique, and Group IV: Restored with methacrylate composite using the vertical layering technique. The samples were stored in distilled water, followed by thermocycling and immersed in 2% methylene blue. The samples were sectioned and evaluated for microleakage at the gingival margin.
Statistical Analysis: Kruskal-Wallis, Fischer exact test, Wilicoxon test, and Mann-Whitney U test.
Results: Silorane composite had significantly lesser microleakage. No significant difference in microleakage was observed above and below the CEJ for Silorane-based composite.
Conclusion: Silorane composite resin showed lesser microleakage compared to methacrylate composite resin.
Clinical Significance: The Silorane-based composites improve the marginal adaptation due to their reduced shrinkage, thereby decreasing the residual stress at the adhesive-tooth interface.
Keywords: Microleakage, polymerization shrinkage, Silorane
|How to cite this article:|
Joseph A, Santhosh L, Hegde J, Panchajanya S, George R. Microleakage evaluation of Silorane-based composite and Methacrylate-based composite in class II box preparations using two different layering techniques: An in vitro study. Indian J Dent Res 2013;24:148
Composites have become very popular in clinical practice due to the increasing demand in aesthetics and continued improvement in technology. But a major drawback of this material is its polymerization shrinkage which is one of the main cause for the loss of marginal integrity.  Direct Class II composite restorations are commonly used in daily practice as they provide good esthetic results at low cost. Chances of failure in adhesive restorations are more commonly seen when the cervical margins are below the cemento-enamel junction (CEJ). 
|How to cite this URL:|
Joseph A, Santhosh L, Hegde J, Panchajanya S, George R. Microleakage evaluation of Silorane-based composite and Methacrylate-based composite in class II box preparations using two different layering techniques: An in vitro study. Indian J Dent Res [serial online] 2013 [cited 2018 Jan 19];24:148. Available from: http://www.ijdr.in/text.asp?2013/24/1/148/114943
Various techniques have been proposed to reduce the polymerization shrinkage stress. A change in the resin monomer constitutes one of the methods to reduce polymerization shrinkage.  A relatively recent development in composites is the replacement of methacrylate monomers with Silorane monomers. The ring-opening chemistry of Silorane resin reduces shrinkage of composites below 1% when compared to the most commonly used methacrylate-based composite which exhibit a polymerization shrinkage of 2-5%.  Silorane-based composites offer two advantages: Low polymerization shrinkage due to the oxirane ring-opening polymerization reaction and increase in hydrophobicity due to the presence of siloxane, promoting the insolubility of the material in the presence of oral fluids. 
Incremental placement of composite resin has been suggested to reduce polymerization shrinkage stress and also to improve marginal adaptation.  Studies have shown that oblique and vertical incremental techniques lower the C-factor value which in turn reduces the polymerization shrinkage stress.  C-factor is the ratio between bonded surfaces versus free surfaces of the cavity. Incremental techniques use less surface area at a time, for bonding, which decreases the C-factor and thereby stress on the adhesive-tooth interface.
Microleakage studies are the most common method of detecting the bond failure along the tooth-restoration interface. Most of the microleakage studies comparing different type of composites have been done on Class V and Class I ,, and very few studies on Class II restorations. 
Hence, an effort has been made in this study to compare the microleakage in Class II box preparations with gingival margin above and below the CEJ using oblique layering and vertical layering techniques with Silorane composites and methacrylate-based composite resin.
| Materials and Methods|| |
Sixty human maxillary premolars (without caries, cracks, or restorations) extracted for orthodontic purposes were gently cleaned to remove both hard and soft deposits and stored in saline.
Standardized Class II mesial and distal simple box preparations were made in each tooth, with the cervical margins of mesial preparation 1 mm above the CEJ and the distal preparation 1 mm below the CEJ. The dimensions of the preparation were standardized to 3 mm buccolingually at occlusal level, 4 mm buccolingually at the gingival floor, and 2 mm mesiodistally. The measurements were verified with a periodontal probe.  Straight fissure diamond abrasive point (SF12, Mani) was used and replaced after every five preparations. The prepared premolar teeth were mounted between two teeth at a time to simulate the proximal contact.
Tofflemire matricing was done in the prepared tooth and wedges were placed. Prior to curing each set of restorations (n = 5), the light irradiance of the curing light was checked with a radiometer.
The prepared teeth were randomly divided into four groups of 15 samples each.
Group I: Oblique incremental technique with Silorane composite
P90 composite (3M ESPE,St. Paul, MN, USA) and its relevant adhesive (LS System Adhesive Primer and Bond, 3M ESPE). The P90 Primer was applied to the enamel and dentin surfaces using a microbrush with agitation for 15 s, gently air-dried and light cured for 10 s at 600 mW/cm 2 using light emitting diode (LED) (LEDition; Ivoclar Vivadent; Liechtenstein). Light cure unit (Ivoclar Vivadent). First increment was a horizontal increment of about 2 mm in depth followed by three oblique layers as shown in [Figure 1]. Light activation was done after each increment according to the manufacturer's instructions for 20 s.
Group II: Vertical incremental technique with Silorane composite
The same procedure as in Group I was followed for placement of the adhesive, and vertical incremental technique was used for restoration as shown in [Figure 2].
Group III: Oblique incremental technique with microhybrid methacrylate-based composite
Clearfil AP-X (Kuraray) and its adhesive (Clearfil SE). The Clearfil SE primer was applied to the entire cavity surface using a microbrush with agitation for 20 s. The primer was gently air-dried to obtain an even film. The Clearfil SE bonding agent was applied to the entire cavity surface and light cured for 10 s. The oblique incremental technique as in Group I was followed and incrementally restored with a methacrylate-based composite (Clearfil AP-X). Light activation was done after each increment according to the manufacturer's instructions for 20 s.
Group IV: Vertical incremental technique with microhybrid methacrylate-based composite
Clearfil SE adhesive system was used as mentioned in Group III. The preparations were restored with a methacrylate-based composite (Clearfil AP-X; Kuraray Co. Ltd., Japan) using the vertical incremental technique as in Group II.
After removing the matrix, all the restorations were further cured from the buccal and lingual aspects. After storing for 24 h in distilled water, all the specimens were subjected to thermocycling at 5-55°C, 30 s dwell time and 5 s transfer time. Apices of teeth were sealed with a layer of sticky wax and all the tooth surfaces were covered with two coats of finger nail polish with the exception of 1 mm around the tooth-restoration interface. The teeth were then immersed in 2% methylene blue dye for 30 min. The teeth were washed thoroughly with distilled water, followed by sectioning mid-sagitally in the mesiodistal plane with a diamond disc (Microdont, Sao Paulo, Brazil) at a slow speed.
One of the two hemisections of each tooth showing the cleanest dye penetration was selected for examination under a stereomicroscope (Olympus) at 20× magnification to assess the extent of microleakage gingivally [Figure 3] and [Figure 4]. The dye penetration was analyzed qualitatively according to 0-4 scale (0- no dye penetration; 1- dye penetration that extended up to one-third of the preparation depth; 2- dye penetration greater than one-third, up to two-third of the preparation depth; 3- dye penetration greater than one-third, up to two-third of the preparation depth; 4- dye penetration past the axial wall).  Two observers scored the degree of dye penetration to avoid any bias.
|Figure 3: Microleakage at tooth-restoration interface with Silorane composite using vertical layering technique, Group II|
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|Figure 4: Microleakage at tooth-restoration interface with methacrylate composite using vertical layering technique, Group IV|
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All statistical testing was performed at a significance level of 0.05. Statistical analysis was performed using Kruskal-Wallis test, Fisher exact test, Mann-Whitney U test, and Wilicoxon Singed rank test.
| Results|| |
[Table 1] shows the different levels of microleakage in four groups studied both above and below the CEJ.
|Table 1: Comparison of levels of microleakage in four groups studied (fisher exact test) |
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Group III showed higher microleakage below CEJ compared to above CEJ [Table 2].
|Table 2: Mean microleakage in groups studied using Kruskal-Wallis test and wilicoxon singed rank test |
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For restorations below CEJ
Group III and Group IV showed significantly higher microleakage compared to Group I and Group II. There was no significant difference between Group I and Group II and also between Group III and Group IV [Table 3].
|Table 3: Pair-wise comparison of mean microleakage in groups using Mann-Whitney U test |
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For restorations above CEJ
Group IV and Group III showed significantly higher microleakage compared to Group I. There was no significant difference between Group I and Group II. Group III and Group IV showed significantly higher microleakage compared to Group II. There was no significant difference between Group I and Group II and between Group III and Group IV [Table 3].
| Discussion|| |
Composite restorative materials represent one of the many successes of modern biomaterials research, as they replace biological tissue both in appearance and function.  The major drawbacks of these materials include polymerization shrinkage, limited toughness, and the presence of unreacted monomers. Composite polymerization always involves some degree of shrinkage depending on the organic matrix.  Silorane resin monomers have been introduced to overcome the drawbacks of polymerization shrinkage of methacrylate-based composites.  The samples were subjected to thermocycling in order to simulate the degradation of bond in the oral cavity. Dye penetration is an excellent method for the determination of marginal leakage in vitro. Methylene blue dye having a molecule size of 1.2 nm 2 is commonly used which is shown to penetrate microgaps.  The increased hydrophilic nature of self-etching adhesives results in increased water and dye absorption to a higher extent than conventional etch and rinse adhesives; hence, 2% methylene blue with storage time of 30 minutes was done. Dye immersion period of 30 min allows only penetration due to capillary action and prevents diffusion of the dye into the adhesive. 
It was found that the Silorane composite restorative material had significantly lesser microleakage than that of Clearfil AP-X, both below and above the CEJ [Figure 5]. Methacrylates contain two carbon-carbon double bonds and can easily form polymers as double bonds are very reactive. During polymerization, the carbon-carbon double bond is broken by the catalyst, the monomers react with each other to form polymers, and the distance between the reacting monomers lessens as intermolecular distance of the monomer molecules in the network shortens from 0.3 nm to 0.15 nm (double bonds are polymerized to covalent main chain bonds).  Although the particles retain their pre-polymerization volume, the reduced distance between the reacting monomers results in volume loss due to shrinkage. The polymerization process of Siloranes occurs via a cationic ring-opening reaction. In contrast to the linear-reactive groups of methacrylate, the ring-opening chemistry of the Siloranes starts with the cleavage and opening of the ring mechanism which helps in gaining space and counteracts the loss of volume which occurs in the subsequent step, when the chemical bonds are formed resulting in reduced polymerization shrinkage. 
In the early stages of the polymerization reaction (pre-gel state), viscous flow compensates for most of the curing contraction by the rearrangement of propagating chains which minimize stresses. At gel-point, termination prevails over chain propagation forming a continuous network, acquiring elastic modulus to resist plastic flow. As polymerization proceeds to the post-gel state, some viscous deformation is still available, but it is not enough to counter balance setting shrinkage and thus stresses are generated.  Siloranes exhibit delay in the attainment of the gel-point under the same curing conditions as dimethacrylate-based composites. Thus, one more reason for Siloranes exhibiting lesser microleakage compared to methacrylates. Similar results of decreased microleakage exhibited by Silorane composites have been found in other studies. ,,
Results of this study showed that there was statistical significance in the microleakage below CEJ and above CEJ restorations in methacrylate-based restorations [Table 2]. This can be attributed to the fact that the gingival seat of a Class II cavity, when prepared below the CEJ, is made up of dentin and cementum. Bonding to dentin has always been poor compared to enamel because of the morphological, histological and compositional differences between the enamel and dentin. Enamel is more mineralized than dentin, having an inorganic content of 96% by weight, whereas the inorganic content of dentin is approximately 70% by weight, 18% organic material, and 12% water. Bonding to dentin is a challenge due to higher water and organic content. Cementum's organic phase consists of coarser collagen fibers than dentin;  hence, a weaker bonding can be expected.
Application of different layering techniques in this study did not show any difference in microleakage. Similar observation was made by some authors in their respective studies. , However, at the same time there are studies, which have proved that different incremental layering techniques have shown reduced microleakage. , Since in vitro studies have a tendency to overestimate and underestimate the in vivo microleakage, more research is necessary to validate the findings in vivo. Further studies on Silorane composites are needed with regard to bond strength, water sorption, color stability subjected to ageing, and mechanical loading as well.
| Conclusion|| |
Within the limits of this study, the following conclusions were drawn:
- Silorane composite showed lower microleakage in comparison to methacrylate composite both above and below the CEJ.
- Methacrylate-based composites showed lower microleakage in above CEJ restorations in comparison to below CEJ restorations when oblique layering was done.
- There was no difference in microleakage between oblique layering and vertical incremental layering technique for both the composites under similar parameters.
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Department of Conservative Dentistry and Endodontics, The Oxford Dental College, Research Centre and Hospital, Bangalore
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2], [Table 3]