Abstract
Objectives
The aim of this study was to compare the mechanical properties of the nano-hydroxyapatite incorporated silorane composite material with the commercially available dental composites.
Methods
Filtek Silorane resin composite was incorporated with 5% and 10% nano-hydroxyapatite crystals and then mechanically tested in comparison along with the commercially available Filtek Silorane and Filtek Supreme XT after 1, 14, 30 and 90 days period.
Results
The mechanical tests revealed that the modified silorane based dental composite had a significant increase in the mechanical properties than the commercially available Filtek Silorane and Filtek Supreme XT.
Significance
The collected data suggests that nano-hydroxyapatite crystals modified silorane may provide the clinicians with a better composite materials having a longer life especially in the posterior restorations where the masticatory forces are very much high.
1
Introduction
The composite materials have been introduced in the early 1970s and their development has been more focused on the filler content and polymerization . Different types of fillers have been used with different products to give it good strength and the property which makes it easy to be polished . The polymerization shrinkage can be lowered by reducing the volume of the matrix .
In the recent years a new silorane-based composite resin has been developed by 3M ESPE, which has a new cationic ring opening monomer systems which is greatly reactive, biocompatible and most importantly that it has a low polymerization shrinkage and can withstand the temperature changes in the oral environment .
The main composition of silorane is the siloxanes and oxiranes, out of which the siloxane makes the backbone and the silorane becomes hydrophobic in nature which is an advantage and provides long term strength intraorally . In silorane the network is formed by the cationic ring opening polymerization of the oxirane which is the main cause of low polymerization shrinkage. When the polymerization begins it starts when the acidic cation open up the oxirane ring and for a new acidic center which is known as carbocation. After the oxirane monomer has been added into it the epoxy ring is opened to attachment of different monomers to form a chain .
Silane increases the hydrophobic property of the surface of the filler and also is an intermediate interface between the filler and resin matrix. Another main function of the silane in silorane is to prevent the attack of silicon and the hydroxyl groups, which may cause any unwanted polymerization process .
Hydroxyapatite is a form of calcium phosphate, which is also the major component of enamel and dentin. It also makes up more than 95% and 60% by weight of the enamel and dentin respectively. Human bone also contain hydroxyapatite in the form of phosphocalcic hydroxyapatite crystals (Ca 10 (PO 3 ) 6 (OH) 2 ) which are biocompatible and bioactive .
The composite material has to have good mechanical properties because they are continuously subjected to forces which are built up due to the high masticatory forces . Flexural strength and compressive strength of a material has to be high so that the filling does not fail when the restoration is subjected to bending loads and also withstand the high compressive forces in the posterior teeth which can go up to 800 N. Another mechanical property which is important is the flexural fatigue limit, which a restoration usually undergoes due to the continuous masticatory load. Cracks once formed, the restorations degrade aggressively due the harsh oral environment . The restorative material should have a high failure point meaning that it should take up the stress up to a very high critical point after which the crack becomes self growing leading to the failure of the restoration. This has been done by introducing filler particles in to the materials which reduce the energy which propagates the crack . These mechanical properties are among the most important criteria for the selection of restorative materials.
As to the best of our knowledge no studies have been done by incorporation nano-HA into silorane and analyzing the mechanical properties. Previously nano-HA was incorporated in dental restorative materials and it was found that the mechanical properties of materials increase with the incorporation of nano-HA . Another recent study done by Chen et al. analyzed experimental composites by modifying them with nano-Ha fibers and saying that good dispersion of the nano-HA crystals at a low loading mass fraction increases the biaxial flexural strength, but at a higher loading of the nano-HA it may act as defect in bundles, leading to the failure of the restorative material . Ilie et al. evaluated the macro, micro and nano properties of silorane composites and concluded that the silorane based composite material had good mechanical properties and is a very stable materials in many solvents mainly alcohol when compared with other methacrylate-based dental composites .
We hypothesize that nano-HA crystals of size < 200 nm should increase the mechanical properties of silorane based dental composites material by incorporating the nano-HA into the composite resin. To test the compressive strength, flexural strength, flexural fatigue limit and fracture toughness the nano-HA crystals were incorporated into the silorane composite material and the mechanically tested. The analysis of variance (ANOVA) was used for statistical analysis of the acquired data. The purpose of this study is to compare the mechanical properties of nano-HA incorporated silorane based material with the commercially available resin based composite.
2
Materials
The composite materials used in the study along with their composition are given in Table 1 . Hydroxyapatite crystals were purchased from Sigma Aldrich (Al-Khobar) [Ca 5 (OH)(PO 4 ) 3 ] nano powder (<200 nm particle size) 97% synthetic, Ethanol [CH 3 CH 2 OH], Analytical grade, 99.5% pure, (molecular weight of 46.07), (Fluka, Germany, Sigma Aldrich).
| Materials | Manufacturer | Type | Filler | Resin matrix |
|---|---|---|---|---|
| Filtek Silorane | 3M ESPE | Micro-hybrid | Silanized quartz, yttriumfluoride 76 wt.% | 3,4-Epoxycyclohexylethylcyclo-polymethylsiloxane, Bis-3,4-Epoxycyclohexylethyl-phenylmethylsilane |
| Filtek Supreme XT | 3M ESPE | Nano-hybrid | Silica nanofiller, zirconia/silica nanocluster; 78.5 wt.% | BIS-GMA, BIS-EMA, urethane dimethacrylate, Triethylene glycol dimethacrylate |
| 5% HA incorporated Filtek Silorane | 3M ESPE + Modification | Micro-hybrid | Silanized quartz, yttriumfluoride 76 wt.%, 5 wt.% HA crystals | 3,4-Epoxycyclohexylethylcyclo-polymethylsiloxane, Bis-3,4-Epoxycyclohexylethyl-phenylmethylsilane |
| 10% HA incorporated Filtek Silorane | 3M ESPE + Modification | Micro-hybrid | Silanized quartz, yttriumfluoride 76 wt.%, 10 wt.% HA crystals | 3,4-Epoxycyclohexylethylcyclo-polymethylsiloxane, Bis-3,4-Epoxycyclohexylethyl-phenylmethylsilane |
The samples were cured by three overlapping irradiations on either side of the specimens with Elipar Free light curing unit (3M ESPE, Dental products, Germany)
2
Materials
The composite materials used in the study along with their composition are given in Table 1 . Hydroxyapatite crystals were purchased from Sigma Aldrich (Al-Khobar) [Ca 5 (OH)(PO 4 ) 3 ] nano powder (<200 nm particle size) 97% synthetic, Ethanol [CH 3 CH 2 OH], Analytical grade, 99.5% pure, (molecular weight of 46.07), (Fluka, Germany, Sigma Aldrich).
| Materials | Manufacturer | Type | Filler | Resin matrix |
|---|---|---|---|---|
| Filtek Silorane | 3M ESPE | Micro-hybrid | Silanized quartz, yttriumfluoride 76 wt.% | 3,4-Epoxycyclohexylethylcyclo-polymethylsiloxane, Bis-3,4-Epoxycyclohexylethyl-phenylmethylsilane |
| Filtek Supreme XT | 3M ESPE | Nano-hybrid | Silica nanofiller, zirconia/silica nanocluster; 78.5 wt.% | BIS-GMA, BIS-EMA, urethane dimethacrylate, Triethylene glycol dimethacrylate |
| 5% HA incorporated Filtek Silorane | 3M ESPE + Modification | Micro-hybrid | Silanized quartz, yttriumfluoride 76 wt.%, 5 wt.% HA crystals | 3,4-Epoxycyclohexylethylcyclo-polymethylsiloxane, Bis-3,4-Epoxycyclohexylethyl-phenylmethylsilane |
| 10% HA incorporated Filtek Silorane | 3M ESPE + Modification | Micro-hybrid | Silanized quartz, yttriumfluoride 76 wt.%, 10 wt.% HA crystals | 3,4-Epoxycyclohexylethylcyclo-polymethylsiloxane, Bis-3,4-Epoxycyclohexylethyl-phenylmethylsilane |
The samples were cured by three overlapping irradiations on either side of the specimens with Elipar Free light curing unit (3M ESPE, Dental products, Germany)
3
Methods
3.1
Preparation of specimens
Light cured dental composites were first dissolved in ethanol, 2 g of Filtek Silorane was solubilized by adding a few drops of ethanol in order to reduce the viscosity of the composite resin in a glass beaker with a magnetic stirrer and then the nano-HA crystals were added in the silorane at 5 wt.% and 10 wt.% with respect to the weight of the resin based composite respectively. These were allowed to run on the magnetic stirrer for 24 h at 37 °C ± 1, to give ample time for ethanol to evaporate in order to obtain the required paste form.
3.2
Fracture toughness analysis
The fracture toughness was calculated by the single-edge notched beam method . A stainless steel split mold with a knife-edged split of 2 mm × 2 mm × 25 mm was slightly overfilled with composite pastes between two glass slides and clamped under pressure for 30 s to allow any excess material to escape. Removing the glass slides, specimens were subsequently polymerized using a visible light curing unit (≈470 nm, Elipar Free light curing unit, 3M ESPE, Dental products, Germany) by overlapping irradiations three times each for 40 s on both sides until the full surface of the specimens had been photo-activated. The cured specimens were then stored in artificial saliva at 37 °C ± 1 for 24 h before the test. A three-point bending test with a span of 20 mm was performed to fracture the specimens at a crosshead speed of 0.25 mm/min on a universal testing machine (Model 8500, Instron Corp., Canton, MA), which was calibrated prior to each testing session. Fracture toughness was calculated based on peak load, length, depth notch and the specimen area . Ten specimens were evaluated in each material after 1, 14, 30 and 90 days of storing in the artificial saliva.
3.3
Flexural strength analysis
Flexural strength was evaluated according to the ISO 4049 specification for polymer-based restorations. 160 specimens of 25 mm × 2 mm × 2 mm were prepared using a stainless steel split mold. The specimens were polymerized as described previously for the fracture toughness test. The cured specimens were stored in artificial saliva at 37 °C ± 1 for 24 h prior to flexural testing and subsequently removed from the artificial saliva, blotted dry and tested wet. A three-point bending test with a span of 20 mm was used to fracture the specimens at a crosshead speed of 0.25 mm/min on universal testing machine (Model 8500, Instron Corp., Canton, MA), which was calibrated prior to each testing session. The bending data were recorded as load to failure. The flexural strength was calculated according to the peak loads, length and the specimen area. Ten specimens were tested in each group on day 1, 14, 30 and 90 days.
3.4
Compressive strength analysis
Compressive strength was evaluated according to the ISO 4049 specification for polymer-based restorations. 160 specimens of 25 mm × 2 mm × 2 mm were prepared using a stainless steel split mold. The specimens were polymerized as described previously for the fracture toughness test. The specimens were stored in 37 °C ± 1 artificial saliva for 24 h. At 24 h, specimen was tested by placement on its longitudinal side on the platens of a universal testing machine (Model 8500, Instron, Canton, MA). The specimens were loaded to failure in compression at a crosshead speed of 1.0 mm/min .
The compressive strength was calculated based on peak load and diameter of the specimen. Ten specimens were evaluated for each material after 1, 14, 30 and 90 days of storing in the artificial saliva.
3.5
Flexural fatigue limit analysis
Flexural fatigue limit was evaluated according to the ISO 4049 specification for polymer-based restorations. The flexural fatigue limit was measured by using a one-way 3 point bending test for flexural fatigue using a universal testing machine (Model 8500, Instron Corp., Canton, MA). The fixtures were separated by a distance of 36 mm and each was of 3 mm in diameter. The composite materials were tested for their flexural fatigue limit at a frequency of 0.5 Hz ( n = 20) at 10 5 cycles under equivalent test conditions. The ‘staircase’ approach method was used for fatigue evaluation. The stress was alternated between the maximum stress and 1 MPa. Tests were conducted in a sequence, with the maximum applied stress in each succeeding test being increased or decreased by a fixed increment, whether the previous test result was a failure or not. The first specimen was tested at approximately 50% of the initial flexural strength value. A vertical load was applied on the center of the specimen to give a proper simulation of the maximum masticatory force. Ten specimens were evaluated for each material after 1, 14, 30 and 90 days of storing in the artificial saliva.
3.6
Statistical analysis
The 1, 14, 30 and 90 days data were subjected to statistical analyses using analysis of variance (ANOVA) and turkey’s post hoc method was used to compare the differences among groups of materials ( P < 0.05) by SPSS software version 18 for windows.
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