Abstract
Objectives
The purpose of this study was to characterize the surface morphology and the elastic properties of four dental restorative flowable composites currently on the market (Venus Diamond Flow, Vertise Flow, Filtex Supreme XT Flow, Surefil SDR Flow). Additionally, one adhesive system (Adhese One F) and one non-flowable composite (Venus Diamond) have also been characterized as the control materials.
Methods
Surface morphology was studied by both scanning electron and atomic force microscopy, and the elastic modulus and the hardness measured by instrumented indentation. Grain analysis was performed on the microscopic images, and statistical analysis was carried out on the results of the nanoindentation measurements.
Results
It was observed that Vertise, Filtek XT and Surefil SDR exhibit stiffness similar to the non-flowable Venus Diamond, whereas Venus Diamond Flow presents itself as the more compliant flowable composite, with Adhese showing intermediate stiffness. Grain analysis of the images confirmed the general rule that the mechanical properties improve with increasing filler loading, with the notable exception of Vertise Flow that shows modulus and hardness as high as 9.1 ± 0.6 and 0.43 ± 0.03 GPa, respectively, for an estimated loading of only ∼40% by volume.
Significance
Whereas generally flowable composites are confirmed not to possess sufficiently strong mechanical properties for bulk restorations, exceptions can eventually be found upon appropriate laboratory screening, as presently seems to be the case for Vertise Flow. However, real practice in actual restorations and respective clinical evaluation are required for final assessment of the suggested results.
1
Introduction
The development of flowable composites appeared in the 1990s as an important advancement in restorative dental materials . Flowables are low viscosity resin composites obtained from formulations with 20–25% lower filler loading than conventional composites . The lower viscosity of flowables makes their placement by injection syringes possible and it limits stickiness. First-generation flowables were used only as liners due to their low elastic modulus. The second-generation flowables developed since 2000 promise increased mechanical properties and are proposed for use in bulk restorations. However, recently the increasingly available clinical reports have not been as successful as expected. For example, post-operative sensitivity did not improved as claimed . Also, sometimes the use of flowables in non-carious cervical lesions did not improve clinical performance, neither when used alone nor as a liner . Therefore, it emerged that the use of flowables in critical stress applications is not recommended in the absence of appropriate preliminary characterization of their mechanical properties and their relationship to surface morphology .
For an evaluation of overall mechanical performance of dental composites no general consensus exists about the most important properties. Commonly, fracture toughness and strength (flexural, uniaxial compressive and tensile) are evaluated . The authors’ focused on Young’s modulus and hardness, which were measured by nanoindentation. Hardness, in particular, is often considered the most effective quantity for both assessing and predicting the performance of dental composites . These elastic properties were determined for four flowables currently used (VDF, VF, FF, SF, full names in Table 1 ) as well as for one positive and one negative control, consisting of a conventional composite for universal restorations and an adhesive system (VD and AO, respectively). Along with the mechanical measurements a morphological analysis of the surface was performed, and the relation between filler distribution and elastic properties discussed.
| Material name | Company | Material type | Matrix type | Filler type | Filler diameter | Filler loading (vol%) |
|---|---|---|---|---|---|---|
| Venus Diamond Flow (VDF) | Heraeus Kulzer | Flowable, nano-hybrid, thixotropic, multifunctional | UDMA, EBADMA | Ba-Al-F silicate glass, YbF 3 , SiO 2 | 20 nm–5 μm | 41 (65% wt) |
| Vertise Flow (VF) | Kerr | Flowable, self-adhering | GPDM, HEMA, a MEHQ a | Prepolymerized particles, Ba glass, colloidal SiO 2 , YbF 3 , ZnO a | 1 μm for Ba glass; nanoscale SiO 2 and YbF 3 . Overall mean: 1 μm | Not specified |
| Filtex Supreme XT Flow (FF) | 3M Espe | Flowable, thixotropic, low wear | BISGMA (10–15% wt), a TEGDMA (10–15% wt), a BISEMA6 (1–5% wt), a functionalized dimethacrylate (1–5% wt) a | Ceramic (52–60% wt), a SiO 2 (3–11% wt), a ZrO x (3–11% wt) a | Not specified | 58–82% wt a |
| Surefil SDR Flow (SF) | Dentsply Caulk | Flowable, fluoride ion release, up to 4 mm thickness, low shrinkage stress | Polymerization modulator, dimethacrylaye resins (<10% wt), a UDMA (<25% wt) a | Ba-B-F-Al silicate glass (<50% wt), a SiO 2 , amorphous (<5% wt), a Sr–Al silicate glass (<50% wt), a TiO 2 (<1% wt) a | Not specified | Not specified |
| Adhese One F (AO) | Ivoclar Vivadent | Adhesive system, self-etch (single step), fluoride ion release, water–alcohol mixture solvent | Non-methacrylate monomer (hydrolithically stable), bis-methacrylamide dihydrogen phosphate (5–25%), a isopropanol 5–15%, a acrylamido sulfonic acid (1–10%), a acrylamido amino acid (5–20%) a | SiO 2 , highly dispersed, a KF (<1%) a | Not specified | Not specified |
| Venus Diamond (VD) | Heraeus Kulzer | Non-flowable, universal, ‘nanocomposite’ (microfill b ) | TCD-DI-HEA, UDMA | Ba-A-F glass, ‘highly discrete’ | 5 nm–20 μm | 64 |
2
Materials and methods
2.1
Specimen preparation
Samples of the six materials, all in A2 shades, were obtained from the respective companies on request. Slabs of ∼10 mm × 10 mm × 2 mm were prepared by filling a Teflon mold and curing the paste through a Mylar sheet. After SEM imaging the samples were polished using a 656 Dimple Grinder (Gatan, USA) with a felt ring loaded with 0.25 μm diameter diamond particles. After polishing the specimens were repeatedly wiped clean with lens paper impregnated with ethanol. All specimens were stored in air before analysis.
2.2
Imaging
SEM micrographs of the composites were acquired with a JSM-6490LA (JEOL, Japan) operating at 15 kV in low vacuum (70–120 Pa) on samples without metal coating, collecting backscattered electrons (BSE). AFM images of the composites were acquired with an MFP-3D (Asylum Research, USA) operating in air in tapping mode with gold coated silicon cantilevers of nominal tip diameter ∼20 nm, spring constant ∼12 N/m and resonance frequency ∼240 kHz (NT-MDT, Russia).
On both SEM and AFM images grain analysis was carried out by Igor Pro 6.0 software (Wavemetrics, USA) dedicated routines.
2.3
Nanoindentation
Nanoindentations were performed with a NanoTest (Micro Materials, UK) equipped with a Berkovich diamond indenter tip with nominal radius ∼50 nm, elastic modulus E t = 1141 GPa, and Poisson’s coefficient ν t = 0.07 . To obtain an accurate indenter area function and to correct for instrument compliance, the system was calibrated according to Ref. . The experiments were performed in controlled temperature (24 ± 0.1 °C) and low noise conditions (signal RMS < 10 mV), under load control, with loading and unloading rates of 0.01 and 0.02 mN/s, respectively, and a 5 s dwell period at maximum load, varying the maximum load between 1 and 5 mN. For each maximum load the indentations were repeated ten times in different regions. Following the Oliver–Pharr method both reduced elastic modulus E r and hardness H could be obtained.
2.4
Statistical analysis
For better assessment of the significance of the observed differences in the mechanical properties of the investigated materials, a statistical analysis of variance (ANOVA) was performed with a significance level of p < 0.01 and Bonferroni post hoc test, by means of the software program Origin 8.0 (Originlab, USA).
2
Materials and methods
2.1
Specimen preparation
Samples of the six materials, all in A2 shades, were obtained from the respective companies on request. Slabs of ∼10 mm × 10 mm × 2 mm were prepared by filling a Teflon mold and curing the paste through a Mylar sheet. After SEM imaging the samples were polished using a 656 Dimple Grinder (Gatan, USA) with a felt ring loaded with 0.25 μm diameter diamond particles. After polishing the specimens were repeatedly wiped clean with lens paper impregnated with ethanol. All specimens were stored in air before analysis.
2.2
Imaging
SEM micrographs of the composites were acquired with a JSM-6490LA (JEOL, Japan) operating at 15 kV in low vacuum (70–120 Pa) on samples without metal coating, collecting backscattered electrons (BSE). AFM images of the composites were acquired with an MFP-3D (Asylum Research, USA) operating in air in tapping mode with gold coated silicon cantilevers of nominal tip diameter ∼20 nm, spring constant ∼12 N/m and resonance frequency ∼240 kHz (NT-MDT, Russia).
On both SEM and AFM images grain analysis was carried out by Igor Pro 6.0 software (Wavemetrics, USA) dedicated routines.
2.3
Nanoindentation
Nanoindentations were performed with a NanoTest (Micro Materials, UK) equipped with a Berkovich diamond indenter tip with nominal radius ∼50 nm, elastic modulus E t = 1141 GPa, and Poisson’s coefficient ν t = 0.07 . To obtain an accurate indenter area function and to correct for instrument compliance, the system was calibrated according to Ref. . The experiments were performed in controlled temperature (24 ± 0.1 °C) and low noise conditions (signal RMS < 10 mV), under load control, with loading and unloading rates of 0.01 and 0.02 mN/s, respectively, and a 5 s dwell period at maximum load, varying the maximum load between 1 and 5 mN. For each maximum load the indentations were repeated ten times in different regions. Following the Oliver–Pharr method both reduced elastic modulus E r and hardness H could be obtained.
2.4
Statistical analysis
For better assessment of the significance of the observed differences in the mechanical properties of the investigated materials, a statistical analysis of variance (ANOVA) was performed with a significance level of p < 0.01 and Bonferroni post hoc test, by means of the software program Origin 8.0 (Originlab, USA).
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