Abstract

INTRODUCTION

One of the major shortcomings of all resin-based composites (RBCs) is that they undergo polymerization shrinkage upon curing. The resulting volumetric contraction produces stress between the bonded restorations and tooth walls, resulting in gap formation and microleakage. This can lead to post-operative sensitivity, recurrent caries, marginal staining, and premature failure of the restoration. Over the years, various strategies have been proposed to minimize the negative effects associated with polymerization shrinkage of RBCs.[1]

A recent introduction in this milieu is the RBC based on silorane chemistry. Silorane-containing resins are obtained from reaction of oxirane and siloxane molecules. They possess two key advantages: polymerization shrinkage lower than 1% due to ring opening during polymerization,[2] and increased hydrophobicity due to the presence of siloxane species. Silorane-based composites have shown to exhibit good mechanical properties comparable to those of clinically successful methacrylate-based composite materials.[3] They have shown to possess less microleakage compared to clinically successful methacrylate-based composites.[4]

Commercially silorane-based RBC is available as Filtek P90 (3M ESPE). It is a microhybrid composite, filled with fine quartz particles, and radio-opaque yttrium fluoride. Filtek P90 is used along with P90 system adhesive, a two step self-etching bonding system. It consists of a P90 system adhesive self-etch primer which is hydrophilic and bonds to the tooth, and a P90 system adhesive bond which is hydrophobic and adheres to the resin.

Although the most reliable conclusions on the performance of bonding systems in the mouth must be derived from long-term clinical trials, laboratory assays are still useful to compare their bonding performance. Bond strength testing is used as a screening tool to help understand and predict the behavior of adhesive systems. There are conflicting reports regarding bond strengths of RBCs bonded to enamel and dentin with self-etch and total-etch adhesives. Most investigations report higher bond strength with total-etch than self-etch adhesives.[5,6] However, recent studies have concluded that self-etch adhesives may produce similar bond strengths as total-etch adhesives.[7–10]

It has also been shown that different light curing unit sources can significantly influence the bond strength of composites to dentin tissue. Currently, four main types of polymerization units are available: QTH units, plasma arc lamps, argon laser units and light emitting diodes. QTH light source consists of a light filtered by a 100 nm bandwidth filter which transmits radiation in the 370-550 nm wavelength range. LEDs produce a narrow band of wavelength (450-490nm) that is situated in the absorption spectrum of camphorquinone, and no filters are required. The LED curing lights have been found to be comparable with the halogen curing light in bond strength studies to dentin.[11] Bond strength has been found to be more dependent on the light source used for dentin bonding agent rather than for curing resin composite.[12] Other authors have suggested that the restorative material itself might be a more critical factor in adhesion than the curing method.[13]

Thus, this study was conducted to evaluate shear bond strength of silorane and bis-GMA based composite resins using total-etch and self-etch adhesive systems, and also to compare the effect of QTH and LED on the shear bond strength of the experimental materials.

MATERIALS AND METHODS

Eighty freshly extracted human molars (forty eight mandibular and thirty two maxillary) were collected and stored in distilled water until use. Routine prophylactic procedure was carried out with rubber cup and pumice slurry for all teeth. Teeth samples were mounted horizontally on acrylic resin block by using a rectangular mould made of aluminium (5×2×2cm) into which self cure acrylic was mixed, followed by the teeth samples at right angle to its long axis. Occlusal enamel of teeth was removed perpendicular to the long axis of each tooth to expose a flat dentin surface at a depth of 1.5 mm, using slow-speed diamond disc with water coolant.

For the composite buildup, a Teflon mould measuring 2 mm in diameter and 3 mm in height was used to build the composite resin cylinder on the dentinal surface in a two-layer increment technique.

Table 1 shows the materials used in the study. They were divided into four groups as follows:

Table 1

Materials used in the study

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Group 1: P90 system adhesive+ Filtek P90- cured with QTH unit

Group 2: P90 system adhesive+ Filtek P90- cured with LED unit

Group 3: Total etch adhesive+ Z100- cured with QTH unit

Group 4: Total etch adhesive+ Z100- cured with LED unit

For Groups 1 and 2, Filtek P90 self-etch primer was applied and left undisturbed for 15 s. It was air blown for 5 s and light cured for 10 s. Filtek P90 Bond was applied, air blown, and light cured using QTH and LED unit for 10 s each. Filtek P90 was placed in increments in the Teflon mould and cured for 40 and 20 s respectively.

For Groups 3 and 4, tooth surface was etched first with 35% phosphoric acid for 15 s, rinsed for 10 s and gently dried with cotton pellet. This was followed by application of two consecutive coats of Adper Singlebond 2 Total Etch adhesive, dried with a gentle stream of air for 5 s, and light-cured using QTH and LED unit for 10 s each. Filtek Z100 was placed in increments in the Teflon mould and cured for 40 and 20 s respectively.

The specimens were stored in distilled water for 24 h at 37°C in humidor. Specimens were then subjected to shear bond strength testing using Instrom Universal testing machine at a crosshead speed of 1 mm per minute using a blade parallel to the adhesive interface between the adhesive and dentin. Values obtained were calculated in Mega Pascal (MPa) peak load at failure divided by the specimen surface area.

The data was subjected to one-way analysis of variance (ANOVA) using SPSS software (Version 11.0). The multiple range test by Tukey's HSD procedure was employed to identify the significance at 5% level if the P-value by one-way ANOVA was significant.

RESULTS

Table 2 summarizes the mean shear bond strengths of all the groups. In the present study, P<0.05 was considered as indicating statistical significance.

Table 2

Mean shear bond strength values of the various groups

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The statistical analysis revealed that there is no significant difference among the shear bond strengths between Groups 3 and 4 (P=1.000). Hence, type of light curing unit is not a significant factor affecting shear bond strength for Z100 resin-based composite using the total-etch technique.

In contrast, the shear bond strength of Group 3 is significantly higher than that of Groups 1 and 2 (P=0.000 for both). Similarly, the bond strength of Group 4 is significantly higher than both Groups 1 and 2 (P=0.000 for both), thus summarizing that total-etch system gives higher shear bond strength values than self-etch system.

The bond strengths for P90 resin-based composites when cured with LED curing unit (Group 2) was significantly lower than when cured with QTH curing unit (Group 1) (P=0.004).

DISCUSSION

The main objective of the bond strength tests is to determine bonding of an adhesive system to the dental hard tissues. One of the commonly used methods to test this adhesion is the shear bond strength testing.[7,8,14] In vitro shear bond strengths are useful and essential for predicting the performance of new adhesive systems in comparison to conventional bonding systems. Total-etch and self-etch bonding systems along with two different curing units were compared for shear bond strength values in the present study.

In the present study, the total-etch adhesive system gave a significantly higher value of shear bond strength compared to the self-etch system, which is in accordance with previous literature.[6,15] Erickson et al compared three self-etch adhesive systems and eleven etch-and-rinse systems by measuring shear bond strength, and concluded that although chemical bonding may be present for some self-etch systems, it does not compete with the bond produced by total-etch systems.[16]

Self-etching priming systems combine etching and priming steps, where the primer is air dried. This results in calcium and phosphate ions being solubilized from apatite crystals, which are suspended in alcohol and water solvents in the primer. When these volatile solvents are evaporated, the concentration of calcium and phosphate may exceed the solubility product constants for calcium phosphate, resulting in its precipitation within the primer. This limits ability of adhesives to penetrate the primed surface, leading to lower bond strength values.[17] On the other hand, there are studies which show that few self-etching systems can give bond strengths comparable to the total-etch technique.[7,14] These variations may be attributed to the type of bonding agent used. Other factors which may influence in vitro bond strength to dentin are the type and age of the teeth, the degree of dentin demineralization, the dentin surface being bonded, the type of bond strength test (shear or tensile), the storage media, and the environmental relative humidity,[18] which are not tested in the present study.

Lower bond strength of P90 with its adhesive system may in part be attributed to differences in the chemistry of composite resins. According to Guiraldo et al, even though there was no difference in light transmission between silorane-and methacrylate-based RBCs, there was a greater degree of subsurface polymerization of the methacrylate-based RBC.[19] In another study, depth of cure and degree of conversion were found to be lower for silorane-based RBCs compared to methacrylate-based RBCs.[20] Subsurface polymerization has been shown to have a relation to the bond strengths.[21] Loading well polymerized composite resin on poorly polymerized layers can lead the restoration to bend inwards, causing marginal fracture, open margins, and cuspal deflection.[22] Guiraldo RD et al showed that Filtek P90 did not present an efficient polymerization in deeper layers, and suggested increasing exposure time or using light curing units with greater irradiance. The lower degree of sub-surface polymerization of Filtek P90 compared to the methacrylate-based composite may be a factor in the lower values of shear bond strengths shown by them.[19]

Composition of RBCs is also crucial in assessing the bond strengths. Miyazaki M et al showed that bond strength increased with increasing filler content. Filtek P90 has a lower filler content of 79% by weight compared to Filtek Z100 (84% by weight). This may in part be responsible for higher bond strength values for methacrylate-based composite resin.[23]

Filtek P90, a packable resin composite, is more rigid than Filtek Z100, a hybrid resin composite. Less rigid materials are more capable of reducing contraction stresses than more rigid materials. Thus, shrinkage stress produced during curing of packable composite resin (Filtek P90) may be greater, reducing adhesion to tooth structure. This factor may have a minor role in the lower bond strengths of Filtek P90, for either light curing method. Amaral et al in a similar study concluded that hybrid resin composites had greater adhesion to dentin than packable resin composites.[13]

It has been shown that different light sources influence restoration bond strength. In the present study, for the methacrylate RBC when used with total-etch adhesive system, there was no significant difference in the shear bond strengths between QTH and LED curing units. Amaral CM et al also found no significant difference in bond strengths using QTH and LED units.[13] Korkmaz Y and Attar N also reported no significant difference in shear bond strength using self-etch adhesives, when cured with either QTH or LED units. The LED unit reduces curing time, which can be considered as an advantage.[11] The generation of radical species for methacrylate curing is produced by camphoroquinone, which decomposes immediately due to exposure of light with a wavelength between 410 and 500 nm to start the polymerization process. QTH and LED units used in this study, with a similar intensity, readily activated the photoinitiator with no significant differences in bond strengths. In a contrasting study, Carvalho et al showed that for total-etch adhesives, polymerization with halogen light gave a higher bond strength value than those obtained with polymerization with LED.[15] D’Alpino et al also reported similar results[12] and attributed it to the lower degree of polymerization with the LED unit used in their study.

In the present study, curing with LED units gave a significantly lower shear bond strength compared to QTH units for the silorane RBC with its adhesive system. For QTH curing unit, many of the emitted photons are out of the absorption spectrum of camphorquinone.[24] Thus the triplex state of camphorquinone is not activated. For LED, the narrow wavelength is within the absorption spectrum of camphorquinone and the specific energy density for camphorquinone is higher, compensating the lower light emitted by these devices. One component of Filtek P90 initiator is camphorquinone, which matches the light spectrum of conventional light sources. In the reaction path, the electron donor acts in a redox process and decomposes iodonium salts into an acidic cation, which starts the ring opening polymerization process. It is hypothesized that the narrow spectrum of LED curing units may not completely activate these multiple initiators in the silorane RBC. QTH curing units with a wider bandwidth may be able to simultaneously activate all components of the P90 initiator. This may account for the lower degree of polymerization with LED unit when compared to the halogen light, thus affecting bond strength. Another reason could be that QTH units have a higher heat generation, which may speed up the polymer chain induction process allowing more monomers to react before curing ends.[25]

CONCLUSION

Within the limits of this study, it can be concluded that total-etch system gave higher shear bond strength values than self-etch system. Type of light curing unit is not a significant factor affecting shear bond strength for methacrylate-based RBCs using total-etch technique. Based on the present results, it can be recommended to use conventional halogen curing units for curing the newly introduced silorane based resin composites. However, this in vitro study needs further in vivo implementation, since factors like pulpal pressure, dentinal fluid may affect dentin bonding. Long-term clinical studies are required to evaluate the efficacy and durability of these bonding agents.

Footnotes

REFERENCES