An In vitro Comparative Evaluation of Microleakage of Silorane based Resin Composite V/s Posterior Nanohybrid Resin Composite in Class II Mesio-occlusodistal (mod) Cavities using Different Restorative Techniques

INTRODUCTION

The advent and widespread use of adhesion has dramatically altered the practice of restorative dentistry. Esthetic restorative materials like composite resins have gained wide popularity during the last few decades due to their predictable adhesion to tooth structure as well as the possibility of providing highly conservative restorations¹ .

Although composite resins were initially developed for use as anterior restorative materials, they are now being routinely used in stress-bearing posterior situations. The incorporation of new monomers, new initiation systems and filler technologies have significantly improved the physical properties of these materials, expanding their use as direct and indirect restorations² .

One major issue of dental composites that has not been successfully addressed till date is their polymerization shrinkage and its associated problems³.

The polymerization shrinkage develops stresses around the tooth restoration interface. Polymerization contraction stresses produce powerful forces that can separate the restoration from the tooth leading to gaps. Lack of sealing allows the occurrence of marginal microleakage⁴ .

Microleakage is defined as the passage of bacteria, fluids or molecules between the cavity wall and the restorative material applied to it. Imperfect margins result in marginal staining and eventually secondary caries⁴ .

Based on the intensive research on shrinkage and stress development, different approaches have been proposed to reduce polymerization shrinkage and the effects of contraction stresses in dental composites⁵ .

One approach has been to modify the composition of the current resin based composites. The ongoing demand for better mechanical and aesthetic properties has promoted the development of several improved direct composite restorative materials like packable composites, nanofilled composites and silorane composites² .

One recent advancement in direct dental restorative materials is the incorporation of nano-technology. A number of nanofilled restorative materials have been produced by various manufacturers with a filler size ranging from 5 to 100 nano meters (nm). Nano filled composite resins have improved aesthetics and mechanical properties² .

Recently, a revolutionary composite material has been introduced which is based on a new matrix system – 'silorane'. Silorane based composites use a novel monomer system derived from the combination of siloxane and oxirane as opposed to methacrylate which is the monomer in all current composite systems. Polymerization of silorane-based composite occurs via a photo cationic ring-opening reaction which results in a lower polymerization contraction compared to the methacrylate based composite⁵ .

A strategy that clinicians have been exploring to overcome polymerization shrinkage and the effects of contraction stresses in composites is the placement of the material using different incremental techniques. Incremental techniques also allow proper reproduction of anatomy and esthetics and prevent marginal overhangs. Of the various incremental placement techniques for Class II restorations, oblique incremental techniques has shown to lower C-factor and reduce shrinkage stresses in wide MOD cavities⁶ .

Another attempt at combating polymerization shrinkage is to employ a strong liner or base that would minimize the bulk of composite as well as reduce polymerization stresses⁷ . Glass ionomer cements may be used as part of an open or closed sandwich restorative protocol. They adhere well to the prepared cavity floors thereby protecting the pulp by sealing the deep dentine in an area where adhesion of the composite may be compromised. In recent times resin modified glass ionomer cements are recommended as the liner under composite restorations. The material's ability to reduce polymerization stresses may be the key to its usefulness in this respect⁷ .

The present study was therefore designed to evaluate the microleakage of posterior nanohybrid composite resin and silorane based composite resin with or without RMGI as liner.

MATERIALS AND METHODS

40 wide Class II MOD cavities were prepared in each human permanent premolar using a cylindrical diamond bur, with occlusal dimension being approximately 2mm. In each specimen, the cervical limit of one proximal box was 1mm below, while the other was 1mm above the cemento-enamel junction. The teeth were randomly divided into four groups containing ten teeth in each group. 

• Group I- Posterior nanohybrid composite resin (Z350 3M Filtek USA) without any liner.
• Group II Posterior nanohybrid composite resin (Z350 3M Filtek USA) with RMGI liner (Fuji II LC GC Japan) 
• Group III Silorane based composite resin (P 90 3M Filtek) without any liner
• Group IV Silorane based composite resin (P 90 3M Filtek) with RMGI liner (Fuji II LC GC Japan)

The samples were then restored with nanohybrid composite resin or silorane based composite. (Fig 1) All the samples were subjected to 1000 thermal cycles of 5⁰ C/55⁰ C with 30 seconds dwell time.

The teeth were then immersed in 2% methelyene blue dye at 23⁰ C for 24 hours. The teeth were sectioned midsagitally in the mesial-distal plane.

The sectioned samples were then examined under stereomicroscope (Lawrence & Mayo India) at 10X magnification to assess the extent of the gingival microleakage.

RESULTS

The results can be summarized as follows; 

Group I (Posterior Nanohybrid composite resin without RMGI liner) showed maximum microleakage scores (Fig 5) 

Group II (Posterior Nanohybrid composite resin with RMGI liner) showed lesser microleakage scores then the group I (Fig 6) Ÿ

Group III (Silorane based composite resin without RMGI liner) showed lesser microleakage scores then the group I and group II (Fig 7) Ÿ

Group IV showed least microleakage scores among the all the groups. (Silorane based composite resin with RMGI liner) (Fig 8)

STATISTICAL ANALYSIS:

The microleakage scores were subjected to Kruskal-wallis test Higher mean microleakage was observed in Group 1 followed by Group 2 and Group 3 respectively. Least mean microleakage was recorded in Group 4. The difference in micro leakage between the groups was found to be statistically significant (P<0.001).

Higher mean microleakage was observed in Group 1 followed by Group 2. Lowest mean micro leakage was recorded in Group 3 and Group 4. The difference in micro leakage between the groups was found to be statistically significant (P<0.05).

DISCUSSION

There has been an exceptional growth in composite resin technology over the past few decades. Some of the promising materials presently available include packable composites, nanohybrid composites and silorane composites. The major advantage with these materials is that polymerization shrinkage is lowered to a significant extent. While nanohybrid composites exhibit polymerization shrinkage upto 1.5%, silorane shrink less than 1% upon polymerization⁵ . Reduction in polymerization shrinkage can greatly improve the marginal adaptation and minimize microleakage.

Nanohybrid composite resins are highly filled. Nano structures are used to produce composites with low shrinkage, high wear resistance and biocompatibility. The inorganic filler loading is 87% by wt and 71.4% by volume. Their volumetric polymerization shrinkage is 1.5% which is much lower than the 2-4% range for hybrid and submicron hybrid composite⁵ .

The name 'silorane' derives from the combination of its chemical building blocks siloxane and oxirane. The siloxane backbone was introduced in order to provide a more hydrophobic nature, which is very important since too high water sorption limits the long term intra-oral physical strength of composites⁴ .

The network of silorane is generated by the cationic ring opening polymerization of cyclo aliphatic oxirane moieties, which accounts for their low shrinkage and low polymerization shrinkage stresses¹ .

Filtek silorane has filler level of 76% and its volumetric shrinkage is less than 1%. The marginal integrity and microleakage of silorane based restorative systems are reported to be superior to that of methacrylate based systems⁵.

Several methods are available to evaluate microleakage under restorations such as: dye penetration, bacterial penetration, electrochemical methods, pressurized fluid measurements, radioisotopic penetration, scintillation spectrometry, etc⁸ . These procedures have been reported to produce varying and often contradictory results. The merits of dye penetration have been discussed extensively and this method still remains as a useful means to test microleakage. 

Ahlberg et al⁹ have reported that methylene blue is a superior tracer of microspaces than Indian ink or procion. In the present study longitudinal sectioning of the teeth were done before evaluating linear dye leakage under a stereomicroscope.

The purpose of the microleakage test was to obtain information about the sealing ability of the restoration adhesive complex4 . In vitro microleakage studies are useful because their results closely match those of in vivo studies since human teeth and clinical protocols are employed.

The results of the present study showed different levels of microleakage for the nanohybrid composite and silorane based composite:

1) Least microleakage was observed for silorane based composite with or without resin modified glass ionomer as liner.

2) Both silorane and nanohybrid composite performed better when RMGI was used as liner.

3) Considerable of leakage was observed at the cemental margin for all the groups.

One approach to minimize the effects of curing shrinkage is the insertion of the resin composite in increments, which lowers the configuration factor⁵ . Of the various incremental placement techniques, the oblique incremental technique has been reported to improve marginal adaptation by resisting resin composite shrinkage stresses⁷ . Oblique incremental technique tends to lower the C-factor, shows better adaptation of and better curing of the composite resin, lesser polymerization shrinkage and less post operative sensitivity.

Hence, in the present study the application of composite resins using oblique incremental technique may be one of the reasons for the reduced microleakage seen with both the silorane and nanohybrid composite resins. 

Incremental resin composite application alone is not enough to prevent or reduce the marginal microleakage. Another attempt to reduce microleakage is the use of RMGI as a liner or base under composite resins. This intermediary layer has been reported to compensate for some of the polymerization shrinkage stresses induced in the composite material ⁷.

The utilization of RMGI liner to control microleakage has been widely studied and compared against several bonding systems. There are many studies demonstrating the superiority of RMGI) over other liners .¹⁰

Compared to the conventional GIC and compomer, RMGICs reach chemical maturation far more rapidly. As a result they are able to resist the early polymerization contraction of the composite resin in a better manner¹⁰ .

In the present study the least microleakage was observed for silorane based composites (group IV) followed by the nanohybrid composite resin (group II). In both these groups RMGI liner was employed.

RMGI liner can be placed either as an open sandwich or as a closed sandwich technique with composite resins. It has been reported that when RMGI is used as a closed sandwich technique, their shrinkage is more than that of the overlying composite resin because of the absence of humidity. Oliveira et al have suggested that under sealed conditions RMGI 11 shrink more than composite resins .Recent reports with RMGI liners have demonstrated that it is now possible to extend it till the cavosurface margin safely and reliably. Dietrich has reported better results for microleakage in Class II composite restorations employing RMGI in an open sandwich technique.¹⁰

In our study also the RMGI liner was placed as an open sandwich technique which could be the reason for better results obtained with groups II and IV.

In the present study the silorane composite when used with or without RMGI liner demonstrated lesser microleakage compared to the nanohybrid composite with or without RMGI liner. This is in concurrence with previous studies which have reported that the marginal integrity and microleakage of silorane based composites are superior to nanohybrid composites based on the methacrylate systems.

The silorane system adhesive is exclusively meant for use with the silorane restorative material and is not recommended for use with any of the methacrylate based systems. It is a two step self etch bonding agent. It may be speculated that this two-step procedure could have improved the quality of the tooth restoration interface⁵ . This has been confirmed in our study where group III (Silorane without RMGI liner) showed better adhesion than group I (Posterior nanohybrid composite resin without RMGI liner) and group II (Posterior nanohybrid composite resin with RMGI liner).

Thus the minimal microleakage seen with silorane groups could be attributed to⁵:

1. The ring opening chemistry of the silorane system.

2. The different nature of the silorane system adhesive.

3.The compatibility between the silorane adhesive and the silorane resin composite.

The greater microleakage with the nanohybrid composite may be due to the higher polymerization shrinkage of methacrylate based composite and the technique sensitivity of total etch adhesive employed with this group. This has also been supported by the findings of Thalaker et al and Palin et al⁵ . In the present study, the methacrylate based nanohybrid composite was bonded using a two-step total –etch adhesive (Adper Prompt).

An interesting finding in our study was that for all the groups when the margins were located on cementum the leakage was significantly higher. Ferrari and Davidson and Ferrari and other described the presence of the outer layer 200-300 micron thick at CEJ which is covered by thin layer of cementum¹² . This layer has been reported to interfere with quality of the bonding as it is hypomineralized and rich in organic matter¹². 

CONCLUSION

Silorane based composite resins showed excellent adaptation when the margins were located on enamel and dentin. Nanohybrid composite resins demonstrated comparably more amount of microleakage when margins were located on enamel and dentin margins. The use of RMGI liner in an open sandwich technique demonstrated significant reduction in microleakage for both silorane and nanohybrid composites. In order to reduce microleakage problems, silorane based composite resins may be better than methacrylate based composite resins. RMGI can be recommended as a liner under composite restorations to minimize microleakage.