In-vitro Comparative Evaluation of Flexural Strength and Depth of Cure of Bulkfill Composite

INTRODUCTION

Dental composites are highly cross-linked polymeric materials reinforced by a dispersion of glass, crystalline or resin filler particles and /or short fibres bound to the matrix by silane coupling agent.Recent advances in their mechanical properties and improved adhesive system have broadened the application of these materials^1,2. However posterior composite resin have limitations like polymerisation shrinkage, shrinkage stress upon curing, cuspal deflection, decreased depth of cure etc.

Recently, a newerformulation ofresin based composite (RBC) material called, “Bulk fill RBC” has been introduced in an attempt to fasten the restorative process. Material thickness of upto5 mmcan be placed and cured in one step without negatively affecting the polymerisation shrinkage, cavity adaptation and degree of conversion. Thus, time consuming layering process can be eliminated^3,4.

But there are concerns regarding the efficacy of greater depth of cure of Bulkfill RBC during their clinical placement. When composite resin is cured in a single bulk layer, there may be a low degree of polymerisation at the deeper portion due to the attenuation of the light. An insufficient degree of curing affects the chemical properties of composite resins and may lead to release of possible allergic or toxic component from the material.

Uncured composite resins at the base of the restoration may cause microleakage which in turn results in pulpal sensitivity,recurrent caries and also staining5 .Additionally, incomplete curing affects the mechanical properties of the material. In the present study, we used two bulkfill composites-Sonic fill and Filtekbulkfill. The Sonicfill system (SF) is arelatively a newbulkfill material which is manufactured by KaVo and Kerr Corporation. It incorporates highly filled proprietary resins with special modifiers that react to sonic energy. Sonic activation lowers the viscosity of this material to upto 87% and allows for easy adaptation to the cavity wall without causing much of the polymerisation shrinkage. Here, the rheological property of the composite is altered without reducing the amount of fillers in it⁴ .

Filtek bulk fill (FBF) is a nanofilled composite which consists of a combination of non-agglomerated and agglomerated nanofiller particles (silica, zirconia, and ytterbium trifluoride fillers). The unique property of FBF composite is that it contains two novel methacrylate monomers that in combination act to lower polymerization stress.

The aim of this study was to investigate the flexural strength and depth of cure of two bulkfill composites (Sonicfill system and Filtekbulkfill) and to compare the results with that of a universal composite (Filtek Z350XT). The null hypothesis to be investigated is that flexural strength and depth of cure of bulk fill resins are similar to that of conventional composite.

MATERIALS AND METHODS

Preparation of thes pecimens for evaluation of flexural strength:

Thirty specimens were prepared using a 25x2x2mm rectangularsplit brass mold.10 specimens each were made withthetwo bulk fill resin composites i.e, Sonicfill nanohybrid bulkfill, Filteknanofilledbulkfill and an universal composite Filtek Z350XT. The inner surface of the mold were coated with petroleum jelly to avoid sticking of the composite.

Curing was done from the top surface 1mm above the resin composite with continuous curing mode using LED (Elipar 3M ESPE) curing unit with three non-overlapping 20sec exposures to cover the entire length of the specimen. The cured specimens were then smoothened with sand paper and polishing disk.

Flexural strength of the specimenswere evaluated by dividing them into 3 groups with 10 specimens each:

Group I : (Control group) Filtek Z350 XT universal composite

Group II : Sonic fill Composite

Group III : Filtek bulk fill Composite Flexural strength was determined usingISO -4049 standard test i.e, the three point bend test with a universal testing machine.Ten specimens from each group were placed on a three point bend test device.Samples were loaded until failure occurred by universal testing machine with a head diameter of 2mm and cross head speed of 0.5mm/min.

Flexural strength was calculated with the formula:3FL/2bd2

F is the maximum load in Newton, L is the distance between the supporting arms in millimetre (20mm), b is the width of the specimen in millimetre and d is the height of the specimen in millimetre.

Preparation of specimens to evaluate the depth of cure

Ten specimens from each group were fabricated using 8*4mm cylindrical spilt brass mold.The inner surface of the mold was coated with petroleum jelly to avoid sticking of the material,mylar strip was pressed over the filled restorative material to obtain a flat surface. Theywere then cured for 20sec with continuous curing mode using LED (Elipar 3M ESPE) from 1mm above the resin composite. The cured samples were smoothened with sand paper and their height was measured in millimetre using digital calliper with 10micrometer resolution.Specimens were divided into 3 groups with 10 specimens each:

Group A : (control group) Filtek Z350 XT universal composite

Group B : Sonic fill Composite

Group C :Filtek bulk fill Composite

Depth of cure of these specimens were determined using Acetone shake test.

Specimens were then placed in the ependorff tubes containing 99.9% pure acetone solution. These tubes were vibrated on a mechanical vibrator for 30seconds.This,vibratory motion removed uncured material from the bottom surface of the specimen leaving the polymerized portion undamaged. The specimens were then removed from the tube, dried and the height of the specimens were measured in millimetre using digital calliper.The remaining sample thickness was divided by 2,the rationale for such subdivision is, the whole specimen will not be optimally polymerised and dividing the remaining sample thickness by two has been considered to provide a safe measure of adequate cure.

RESULTS:

The analysed data of mean FS values of different composite resin materials are presented in table 1and graph 1.The Flexural strength value ranged from 133.28MPa to 154 MPa with the highest value for the SonicFill composite (group II) and least for the universal composite, Filtek Z350XT (Group I).

Post – hoc comparison using Tukey’s test showed that there was statistically high significant (p<0.001) difference in FS values between Sonic fill and Filtek Z350XT universal composite. Whereas, comparison between Filtekbulkfill and Filtek Z350 XT showed no statistically significant difference in FS value (p=0.393).

Between the two bulkfill composites, SonicFill had significantly higher FS value that that of Filtekbulkfill composite (P = 0.003)

The mean values of DOC of different composites are shown in table 2 and graph 2. Results showed that both the experimental groups had higher DOC values than that of control group. Here,Filtekbulkfill composite (group C) achieved highest (3.94mm) value of DOC and Filtek Z350XT universal composite (group A) showed the least (2.24mm). Multiple comparison among the tested group showed that there was statistically high significant (p<0.001) difference in depth of cure values between bulkfill composite and control group (p<0.001).

Also within the bulkfill composites, there was highly significant difference in DOC values between SonicFilland Filtekbulkfillcomposites .

DISCUSSION:

There have been various advancementsin resin matrix system and filler technology to reduce the polymerization shrinkage and improve the mechanical properties such as hardness, compressive strength, flexural strength and fracture toughness² .

However low degree of monomer conversion is still a major disadvantage of many composites6. Because of this inadequate curing, an incremental placement technique of maximum of 2mm thickness of composites is generally practiced⁷ . But, this incremental placement technique delayed the restorative process and also there may be a chance of moisture contamination, air bubble entrapment between the increments.

To overcome the disadvantages of incremental build up a new class of composite called “Bulkfill resin composite” has been introduced for posterior restorations. These composites can be filled and cured upto5 mm thickness in a single increment without affecting polymerization shrinkage, degree of conversion and cavity adaptation5. These BFC have higher filler volume percentages and modified initiator systems to enable better curing depth, thereby providing superior physical, mechanical properties and reduced cuspal deflection. Moreover, the rheology of these materials is also changed, thereby allowing a better adaption to the cavity walls.

The present study evaluated two bulkfill and one universal composite, which are nanofilled in nature. Nano composites were selected because of their low polymerisation shrinkage, good mechanical properties, high wear resistance and excellent polish retention owing to their high filler content.

Sonicfill BFC are highly filled resins with special modifiers that react to sonic energy. Sonic activation lowers the viscosity of the materialupto 87% and allows for easier adaptation to the cavity wall without causing much of the polymerisation shrinkage⁵ .

After placement, the material will return to a nonslumping state which helps in easier contouring of the material and does not require an additional capping layer. So, here the flowability of the material is improved without reducing the amount of fillers present in it. Hence, has satisfactory physicomechanical properties, suggesting their use as a direct posterior composite resin restorative material.

Filtek Bulk Fill (FBF) posterior restorative material is ananofilled composite resin used to make posterior restorations simpler and faster. This bulk fill material provides excellent strength and low wear. FBF is available in traditional syringes and single-dose capsules. Due to its excellent polish retention properties, it is also suitable for anterior restorations. The fillers of these bulkfill composites are combination of non-agglomerated 20 nm silica filler, non-agglomerated 4 to 11 nm zirconia filler, aggregated zirconia/silica cluster filler and a ytterbium trifluoride filler. The inorganic filler loading is about 76.5% by weight (58.4% by volume). It contains AUDMA, UDMA and 1, 12-dodecane-DMA as monomers in the resin matrix. The presence of special methacrylate groups in the monomer, helps in reducing the polymerization shrinkage stress^8,9,10.

According to Anusavice, flexural strength (FS) is defined as the failure stress of a material as measured in bending. Clinically, composite resin restorations are indicated in areas of stress. So, these materials will be subjected to considerable amount of flexural stresses and high FS is necessary to withstand biting forces without undergoing fracture.

In the present study SF (group II) composite achieved higher FS values than that of FBF composite (group III). This result was in accordance with the previous study done by Ilie et al. This is due to increased amount of inorganic fillers present in this composite. Studies have explained positive correlation between the amount of fillers and FS value. Though F350 (group I) had higher filler loading than FBF composite, it produced lesser FS value. This could be due to TEGDMA and PEGDMA in the resin matrix and the characteristic flexibility of these monomers causes the creation of flexible polymer network that increases the elastic deformation of this composite. According to ISO, all the polymer based restorative materials should have minimum FS value of 80Mpa to be used in stress bearing area.In the present study all the tested materials produced higher FS values than this, thereby justifying their usage in stress bearing areas¹¹.

The depthof cure of visible light activated dental resins has been recognized as an important factor to the clinical success of these materials. Several factors such as composition, shade (opacity), type of photo initiator, wave length, light intensity, the distance between the cured mass and the curing tip and the thickness of the cured increment can affect the curing efficiency of the RBC⁶ . A group of methods such as hardness test, translucency changes, interaction with colour dyes, double bond conversion, micro imaging, tactile tests, penetration and scraping tests have been used to measure the depth of cure of composites.

In the present study, we used an innovative method of determining depth of cure using acetone shake test. Since pure acetone or ethanol solutions have the ability to dissolve the uncured monomers. In acetone shake test, the composite samples are placed in Ependorff tubes containing pure acetone solution and vibrated in mixing device for 30 seconds. The uncured portion werewashed out and remaining cured part was measured to determine the DOC. The dimension of remaining hardened portion was measured and divided by two, similar to that of scrape test.

In the present study FBF( group C) produced statistically significant higher value of depth of cure(3.94mm) than that of SF(3.52mm) and F350 universal composite (control group) (p<0.001). This is because of the fact that FBF have decreased filler loading than that of other composites which accounts for its improved translucency. Moreover refractive indices of resin matrix and filler particles are almost similar which causes less light scattering at their interface permitting the light to enter deeper2,13. In this study, SF and FBF produced only 3.52mm and 3.94mm depth of cure respectively. The DOC of SF composite was similar to that of the results obtained by Vandewalle et.al and Benetti et. al using scrape test.

Although in the present study, acetone shake test produced similar value of depth of cure as that of ISO scrape test. There are no previous studies comparing these tests and therefore further laboratory investigations have to be conducted comparing the results between these tests.

Brass mold used in this study, could have reflected the lightwhich could be one of the reasons for the decreased DOC values of all the tested materials than that claimed by the manufacturer.

In our study, Elipar S 10 LED ( 3M ESPE) second generation monowave, light curing unit was used for curing the composite material. The narrow spectrum of monowave LED light curing units might not have optimally cured the bulkfill composites that incorporates multiple photoinitiators with varying peak of absorption ranges^16,17.

The present study showed that all the tested materials are suitable and acceptable for restorations involving occlusal surfaces.

The bulk-fill technology has obvious advantages: 1) Fewer voids present in the mass of the material, since it is placed at one time 2) The technique would be faster than placing numerous increments. 3) Lesser exposure to the intense curing light.

CONCLUSION:

Within the limitation of this study it can be concluded that:-

• All the tested composite resin materials showed significantly higher flexural strength value than the minimum flexural strength (80MPa) that is required to use in stress bearing areas.

• Sonic fill composite produced highest value of flexural strength than other composites due to its increased percentage of filler content.

• Acetone shake test can be considered as one of the laboratory test to investigate the depth of cure of resin composites.

Further studies evaluating the long term clinical success of bulk fill resin based composites are needed.