Chemical characterization and bioactivity of epoxy resin and Portland cement-based sealers with niobium and zirconium oxide radiopacifiers

Abstract

Objective

The purpose of this study was to characterize and to evaluate the bioactivity potential of experimental root canal sealers (ES) based on Portland cement, epoxy resin with nano- and micro-particles of niobium or zirconium oxide used as radiopacifiers in comparison to AH Plus and MTA Fillapex.

Methods

Specimens of the sealers (10 mm in diameter × 1 mm thick) were prepared and the radiopacity was evaluated according to ISO 6876 (2012) specifications. Characterization of the sealers was performed under the scanning electron microscope (SEM) immediately after setting and after immersion for 28 days in Hank’s balanced salt solution (HBSS). In addition X-ray energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy were also performed. The pH and calcium ion release were measured after 1, 7, 14, 21 and 28 days after completion of seating using a digital pH meter and an atomic absorption spectrophotometer, respectively.

Results

The experimental sealers exhibited an average radiopacity of 2.5 mm thickness of aluminum, which was similar to MTA Fillapex ( P > 0.05) and inferior to AH Plus ( P < 0.05). AH Plus did not show bioactivity. Although the experimental sealers did not exhibit the formation of hydration product, they encouraged the deposition of crystalline spherical structures of calcium deficient phosphate. The highest pH and calcium release values were observed with the experimental sealers ( P < 0.01). ES-Nb-micro was the only sealer to present hexagonal shaped crystal deposition.

Significance

Novel root canal sealers based on a mixture of Portland cement, epoxy resin and radiopacifier exhibited a degree of bioactivity although no evidence of cement hydration was demonstrated on material characterization. The radiopacifier particle size had limited effect on the sealer microstructure and chemical properties.

Introduction

Bioactivity can be defined as a beneficial effect produced by some materials when they are implanted in living tissues. Through biochemical and biophysical reaction, the tissue fluid interacts with these materials leading the formation of carbonated apatite crystals, which are the main mineral phase of hard tissues, such as bone, dentin and cementum . Ideally, root canal sealers and root-end filling materials should be bioactive since they are in directly contact with periapical tissues through the root apex.

Mineral trioxide aggregate (MTA) is used extensively in Dentistry mainly as reparative cement in cases of root perforation , pulpotomies and as a root-end filling material due to its biological properties. MTA induces hard tissue deposition, and is thus bioactive . The bioactivity results from the reaction of the calcium hydroxide produced during the hydration of the Portland cement component with phosphates present in tissue fluids . Most root canal sealer cements do not possess any bioactivity. MTA Fillapex (Angelus, Londrina, Brazil) which is a sealer based on MTA, was reported to exhibit bioactivity when in contact with simulated tissue fluids in human cell culture .

MTA and Portland cement mixed with water, results in a granular and sandy paste with unfavourable handling characteristics that precludes the use of MTA as a root canal sealer . Therefore, a number of studies have been performed attempting to develop a root canal sealer based on MTA or tricalcium silicate , which is the main constituent phase in MTA. MTA Fillapex is composed of MTA and other compounds, such as resins, that result in an endodontic material with adequate physicochemical properties to be used as sealer . Since MTA and Portland present a similar chemical composition and biological response , an experimental root canal sealer (MTA Sealer), containing white Portland cement, a radiopacifying agent (zirconium oxide), an additive (calcium chloride) and a resinous vehicle, which conferred viscosity to the sealer, have been developed and its physicochemical and biological properties showed promising results .

The role played by radiopacifying agents in the bioactivity of the endodontic materials is still not well reported. Bismuth oxide is the radiopacifying agent present in MTA Fillapex and some studies have shown that this material in general, negatively affects the physico-chemical and biological properties of MTA cements. Furthermore bismuth oxide has been implicated with tooth discoloration by interacting with collagen present in dental hard tissues and also reacting with sodium hypochlorite used routinely during endodontic therapy . To avoid the side effects caused by bismuth oxide, alternative radiopacifying agents have been proposed . Replacement of bismuth oxide with zirconium oxide resulted in a material with physicochemical properties comparable to the commercial version that contained bismuth oxide . Furthermore, this material was shown to be bioactive since it induced the deposition of precipitates that precedes apatite formation when in contact with simulated body fluid .

Niobium is a transition metal that can also be added to root canal sealers to enhance radiopacity. Niobium oxide increased the radiopacity of methacrylate-based root canal sealers . Also, the oxidized form exhibited biocompatibility and ability to enucleate hydroxyapatite when it was used to cover dental implants .

The purpose of this study was to characterize and evaluate the bioactivity potential of experimental root canal sealers based on Portland cement and an epoxy resin incorporating nano and micro particles of niobium or zirconium oxide radiopacifiers and compare these novel sealers to AH Plus and MTA Fillapex.

Methodology

The materials used in this study included a range of conventional and experimental (ES) root canal sealers:

  • AH Plus (Dentsply International, Addlestone, UK);

  • MTA Fillapex (Angelus Dental Solutions, Londrina, SP, Brazil);

  • ES-Zr-micro (Araraquara Dental School, São Paulo State University, Brazil) composed of a mixture of Portland cement, micro-sized zirconium oxide (Sigma–Aldrich, St Louis, MO) and an epoxy resin;

  • ES-Zr-nano (Araraquara Dental School, São Paulo State University, Brazil) composed of a mixture of Portland cement, nano-sized zirconium oxide (Institute of Physics of São Carlos, University of São Paulo, São Carlos, Brazil) and an epoxy resin;

  • ES-Nb-micro (Araraquara Dental School, São Paulo State University, Brazil) composed of a mixture of Portland cement, micro-sized niobium oxide (CBMM, Companhia Brasileira de Metalurgia e Mineração, Araxá, MG, Brazil) and an epoxy resin;

  • ES-Nb-nano (Araraquara Dental School, São Paulo State University, Brazil) composed of a mixture of Portland cement, nano-sized niobium oxide (Institute of Physics of São Carlos, University of São Paulo, São Carlos, Brazil) and an epoxy resin.

The radiopacifiers replaced 30 wt% of white Portland cement (Portland Cement; CPB-40; Votorantin Cimentos, Camargo Correa S.A., Pedro Leopoldo, MG, Brazil). The micro-particles of niobium and zirconium oxide were purchased from companies manufacturing chemicals. The nanoparticles of niobium and zirconium oxide were prepared by the polymeric precursor method. The zirconium oxide supports were prepared from the precursor salt ZrO(NO 3 ) 2 · x H 2 O (Alfa Aesar). Aqueous solutions of this salt were prepared, mixed and added to an aqueous solution of citric acid (held at 60 °C), with constant stirring. Subsequently, ethylene glycol (HOCH 2 CH 2 OH) was added to polymerize the citrate by a polyesterification reaction (at 120 °C). The citric acid:metal molar ratio was 3:1, while the citric acid:ethylene glycol mass ratio was 60:40. The resulting polymer resin was then calcined at 300 °C for 4 h, and the after 600 °C/2 h to produce ZrO 2 crystalline particles. Whereas to produce niobium oxide nanoparticles, an aqueous solution of niobium ammonium oxalate {NH 4 [NbO(C 2 O 4 ) 2 (H 2 O)](H 2 O)N (CBMM, Companhia Brasileira de Metalurgia e Mineração, Araxá, MG, Brazil) was prepared and ammonium hydroxide was dropped upon thereafter. The niobium hydroxide precipitated was filtered and washed to eliminate oxalate ions and dissolved into a citric acid (CA) aqueous solution ([CA]/[Nb] = 3) and filtered. The niobium content in the solution was precisely determined by gravimetric analysis. The solution was stirred for 2 h at 70 °C to promote the complex reaction. Ethylene glycol (EG) was added to the mixture with mass ratio was 60:40. The translucent solution was heated and stirred over several hours. A polymerization process started during the water evaporation, resulting in a highly viscous solution. This resin was heated in an electric furnace at 300 °C for 4 h. The resulting black and soft mass was milled and calcined in an electric furnace for 2 h over alumina slabs at 700 °C/2 h.

The experimental sealers were mixed with an epoxy resin, composed of equal amounts of catalyst and base pastes which were mixed in a powder/liquid ratio of 5:3 (mass) for the materials containing micro-particles (ES-Zr-micro and ES-Nb-micro) and 5:4 for those containing nanoparticles (ES-Zr-nano and ES-Nb-nano). The powder/liquid ratio of the sealers containing nanoparticles was determined by a previous pilot study. AH Plus (Dentsply, De Trey, Konstanz, Germany) and MTA Fillapex (Angelus, Londrina, PR, Brazil) were used as controls and mixed according to manufacturer’s instructions.

Evaluation of radiopacity

Radiopacity evaluation of the set sealers and the raw materials making up the prototype sealers was performed using ISO 6876:2012 recommendations . Three specimens 10 ± 1 mm in diameter and 1 ± 0.1 mm thick were used. The powders were compacted in circular molds of similar dimensions. The specimens were radiographed by placing them directly on a photo-stimulable phosphor (PSP) plate adjacent to a calibrated aluminium step wedge (Everything X-ray, High Wycombe, UK) with 3 mm increments. A standard X-ray machine (GEC Medical Equipment Ltd., Middlesex, UK) was used to irradiate X-rays onto the specimens using an exposure time of 1.60 s at 10 mA, tube voltage at 65 ± 5 kV and a cathode-target film distance of 300 ± 10 mm. The radiographs were processed (Clarimat 300, Gendex Dental Systems, Medivance Instruments Ltd., London, UK) and a digital image of the radiograph was obtained. The grey pixel value on the radiograph, of each step in the step-wedge was determined using an imaging program, Microsoft Paint (Microsoft Corp., Redmond, WA, USA) as a number between 0 and 255 with 0 representing pure black and 255 pure white. A graph of thickness of aluminum vs. grey pixel value on the radiograph was then plotted and the best-fit logarithmic trend line was plotted through the points. The equation of the trend line gave the grey pixel value of an object on the image as a function of the object’s thickness in mm of aluminum. This equation was inverted so as to express the object’s thickness as a function of its grey pixel value on the radiograph. The grey pixel values of the cement specimens were then determined using the imaging program, and the equivalent radiopacity of the cement sample, expressed in mm of aluminium was thus calculated.

Characterization of raw materials

The Portland cement, micro zirconium and niobium oxide were characterized using a combination of scanning electron microscopy (SEM) and X-ray energy dispersive analysis (EDX), and X-ray diffraction (XRD) analysis.

Scanning electron microscopy and X-ray energy dispersive analysis

The powders were impregnated in resin (Epoxyfix, Struers GmbH, Ballerup, Denmark) under vacuum. The resin blocks were then ground using progressively finer diamond discs and pastes using an automatic polishing machine (Tegramin 20, Struers GmbH, Ballerup, Denmark). The specimens were mounted on aluminum stubs, carbon coated and viewed under the scanning electron microscope (SEM; Leo 1430 Oxford, Cambridge, UK). Scanning electron micrographs of the different material microstructural components at different magnifications in back-scatter electron mode were captured and X-ray energy dispersive analysis (EDX) of the different phases was carried out.

X-ray diffraction analysis

Phase analysis of unreacted powders was carried out using X-ray diffraction. The diffractometer (Bruker D8 Advance, Bruker Corp., Billerica, MA, USA) used Cu Kα radiation at 40 mA and 45 kV. Samples were presented in powder form and the detector was rotated between 15° and 45°. A step of 0.02° 2 θ and a step time of 1 s were used. For the powdered specimens the sample holder was spun at 15 rpm. Phase identification was accomplished using a search-match software utilizing ICDD database (International Centre for Diffraction Data, Newtown Square, PA, USA).

Characterization of set materials

The set sealers were characterized using a combination of scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) analysis and Fourier transform infrared (FT-IR) spectroscopy. The characterization was performed on freshly prepared materials and also on sealers that had been immersed in 7 mL of Hank’s balanced salt solution (HBSS; H6648, Sigma–Aldrich, St. Louis, MO, USA) at 37 °C for 28 days. In addition, the surface morphology of the set sealers after 28-day of immersion in HBSS was assessed by scanning electron microscopy.

Scanning electron microscopy and X-ray energy dispersive analysis

Cylindrical specimens (10 mm in diameter and 2 mm thick) were prepared for the set cements. They were divided into three groups. Group 1 was allowed to set for 24 h at 37 ± 1 °C. Group 2 and 3 were allowed to set for 24 h at 37 ± 1 °C after which they were immersed in HBSS for 28 days. Both group 1 and 2 were impregnated in resin (Epoxyfix, Struers GmbH, Ballerup, Denmark) under vacuum. The resin blocks (groups 1 and 2) were then ground using progressively finer diamond discs and pastes using an automatic polishing machine (Tegramin 20, Struers GmbH, Ballerup, Denmark). The specimens in Group 3 were not embedded in resin and were dried in a desiccator with silica gel. All specimens were mounted on aluminum stubs, carbon coated and viewed under the scanning electron microscope (SEM; Leo 1430 Oxford, Cambridge, UK). Scanning electron micrographs of the different material microstructural components at different magnifications in back-scatter electron mode for Groups 1 and 2 were captured and X-ray energy dispersive spectroscopy (EDS) of the different phases was carried out. Group 3 was viewed using secondary electron imaging.

X-ray diffraction analysis

The set sealers were crushed using a mortar and pestle immediately after setting and after 28 days of immersion in HBSS. X-ray diffraction analysis was performed using the same parameters set for the raw materials.

Fourier transform infra-red spectroscopy

The composition of set sealers was investigated immediately after setting and after 28 days of immersion in HBSS using Fourier transform infrared (FT-IR) spectroscopy. To obtain the FT-IR spectrums from powder compounds and from the mixed material immediately after setting or after 28 days in HBSS, 2–5 mg of each powder component or crushed mixed samples of the sealers was added to 100 mg potassium bromide and analyzed in the IR spectrophotometer (Shimadzu IRAffinity-1; Shimadzu Corp., Kyoto, Japan) using transmitted infrared spectroscopy.

pH and calcium ion release

Polyethylene tubes ( n = 10) measuring 10 mm in length and 1 mm of internal diameter were filled with freshly prepared sealers and immersed in 10 mL distilled water. The specimens were stored at 37 °C throughout the experiment. Distilled water (10 mL) served as negative control. pH and calcium ion release were performed after 1, 7, 14, 21 and 28 days. At each time point the elution was collected for testing and solutions were replaced by fresh distilled water. The pH was measured using a previously calibrated digital pH meter (Digimed, Santo Amaro, SP, Brazil). The calcium ion release was assessed using an atomic absorption spectrophotometer (H1170 Hilger & Watts; Rank Precision Industries Ltd. Analytical Division, London, UK). The concentration of calcium ions released from the materials was quantified using a calcium hollow cathode lamp (422.7-nm wavelength and 0.7-nm window) operated at 20 mA. The readings of calcium ion release were compared with a standard curve obtained from multiple dilutions of pure calcium in ultrapure water.

Statistical analysis

The data were evaluated using SPSS (Statistical Package for the Social Sciences) software (PASW Statistics 18; SPSS Inc., Chicago Illinois, USA). Parametric tests were performed as K-S tests on the results indicated that the data were normally distributed. Analysis of variance (ANOVA) with P = 0.05 and Tukey post-hoc test were used to perform multiple comparison tests.

Methodology

The materials used in this study included a range of conventional and experimental (ES) root canal sealers:

  • AH Plus (Dentsply International, Addlestone, UK);

  • MTA Fillapex (Angelus Dental Solutions, Londrina, SP, Brazil);

  • ES-Zr-micro (Araraquara Dental School, São Paulo State University, Brazil) composed of a mixture of Portland cement, micro-sized zirconium oxide (Sigma–Aldrich, St Louis, MO) and an epoxy resin;

  • ES-Zr-nano (Araraquara Dental School, São Paulo State University, Brazil) composed of a mixture of Portland cement, nano-sized zirconium oxide (Institute of Physics of São Carlos, University of São Paulo, São Carlos, Brazil) and an epoxy resin;

  • ES-Nb-micro (Araraquara Dental School, São Paulo State University, Brazil) composed of a mixture of Portland cement, micro-sized niobium oxide (CBMM, Companhia Brasileira de Metalurgia e Mineração, Araxá, MG, Brazil) and an epoxy resin;

  • ES-Nb-nano (Araraquara Dental School, São Paulo State University, Brazil) composed of a mixture of Portland cement, nano-sized niobium oxide (Institute of Physics of São Carlos, University of São Paulo, São Carlos, Brazil) and an epoxy resin.

The radiopacifiers replaced 30 wt% of white Portland cement (Portland Cement; CPB-40; Votorantin Cimentos, Camargo Correa S.A., Pedro Leopoldo, MG, Brazil). The micro-particles of niobium and zirconium oxide were purchased from companies manufacturing chemicals. The nanoparticles of niobium and zirconium oxide were prepared by the polymeric precursor method. The zirconium oxide supports were prepared from the precursor salt ZrO(NO 3 ) 2 · x H 2 O (Alfa Aesar). Aqueous solutions of this salt were prepared, mixed and added to an aqueous solution of citric acid (held at 60 °C), with constant stirring. Subsequently, ethylene glycol (HOCH 2 CH 2 OH) was added to polymerize the citrate by a polyesterification reaction (at 120 °C). The citric acid:metal molar ratio was 3:1, while the citric acid:ethylene glycol mass ratio was 60:40. The resulting polymer resin was then calcined at 300 °C for 4 h, and the after 600 °C/2 h to produce ZrO 2 crystalline particles. Whereas to produce niobium oxide nanoparticles, an aqueous solution of niobium ammonium oxalate {NH 4 [NbO(C 2 O 4 ) 2 (H 2 O)](H 2 O)N (CBMM, Companhia Brasileira de Metalurgia e Mineração, Araxá, MG, Brazil) was prepared and ammonium hydroxide was dropped upon thereafter. The niobium hydroxide precipitated was filtered and washed to eliminate oxalate ions and dissolved into a citric acid (CA) aqueous solution ([CA]/[Nb] = 3) and filtered. The niobium content in the solution was precisely determined by gravimetric analysis. The solution was stirred for 2 h at 70 °C to promote the complex reaction. Ethylene glycol (EG) was added to the mixture with mass ratio was 60:40. The translucent solution was heated and stirred over several hours. A polymerization process started during the water evaporation, resulting in a highly viscous solution. This resin was heated in an electric furnace at 300 °C for 4 h. The resulting black and soft mass was milled and calcined in an electric furnace for 2 h over alumina slabs at 700 °C/2 h.

The experimental sealers were mixed with an epoxy resin, composed of equal amounts of catalyst and base pastes which were mixed in a powder/liquid ratio of 5:3 (mass) for the materials containing micro-particles (ES-Zr-micro and ES-Nb-micro) and 5:4 for those containing nanoparticles (ES-Zr-nano and ES-Nb-nano). The powder/liquid ratio of the sealers containing nanoparticles was determined by a previous pilot study. AH Plus (Dentsply, De Trey, Konstanz, Germany) and MTA Fillapex (Angelus, Londrina, PR, Brazil) were used as controls and mixed according to manufacturer’s instructions.

Evaluation of radiopacity

Radiopacity evaluation of the set sealers and the raw materials making up the prototype sealers was performed using ISO 6876:2012 recommendations . Three specimens 10 ± 1 mm in diameter and 1 ± 0.1 mm thick were used. The powders were compacted in circular molds of similar dimensions. The specimens were radiographed by placing them directly on a photo-stimulable phosphor (PSP) plate adjacent to a calibrated aluminium step wedge (Everything X-ray, High Wycombe, UK) with 3 mm increments. A standard X-ray machine (GEC Medical Equipment Ltd., Middlesex, UK) was used to irradiate X-rays onto the specimens using an exposure time of 1.60 s at 10 mA, tube voltage at 65 ± 5 kV and a cathode-target film distance of 300 ± 10 mm. The radiographs were processed (Clarimat 300, Gendex Dental Systems, Medivance Instruments Ltd., London, UK) and a digital image of the radiograph was obtained. The grey pixel value on the radiograph, of each step in the step-wedge was determined using an imaging program, Microsoft Paint (Microsoft Corp., Redmond, WA, USA) as a number between 0 and 255 with 0 representing pure black and 255 pure white. A graph of thickness of aluminum vs. grey pixel value on the radiograph was then plotted and the best-fit logarithmic trend line was plotted through the points. The equation of the trend line gave the grey pixel value of an object on the image as a function of the object’s thickness in mm of aluminum. This equation was inverted so as to express the object’s thickness as a function of its grey pixel value on the radiograph. The grey pixel values of the cement specimens were then determined using the imaging program, and the equivalent radiopacity of the cement sample, expressed in mm of aluminium was thus calculated.

Characterization of raw materials

The Portland cement, micro zirconium and niobium oxide were characterized using a combination of scanning electron microscopy (SEM) and X-ray energy dispersive analysis (EDX), and X-ray diffraction (XRD) analysis.

Scanning electron microscopy and X-ray energy dispersive analysis

The powders were impregnated in resin (Epoxyfix, Struers GmbH, Ballerup, Denmark) under vacuum. The resin blocks were then ground using progressively finer diamond discs and pastes using an automatic polishing machine (Tegramin 20, Struers GmbH, Ballerup, Denmark). The specimens were mounted on aluminum stubs, carbon coated and viewed under the scanning electron microscope (SEM; Leo 1430 Oxford, Cambridge, UK). Scanning electron micrographs of the different material microstructural components at different magnifications in back-scatter electron mode were captured and X-ray energy dispersive analysis (EDX) of the different phases was carried out.

X-ray diffraction analysis

Phase analysis of unreacted powders was carried out using X-ray diffraction. The diffractometer (Bruker D8 Advance, Bruker Corp., Billerica, MA, USA) used Cu Kα radiation at 40 mA and 45 kV. Samples were presented in powder form and the detector was rotated between 15° and 45°. A step of 0.02° 2 θ and a step time of 1 s were used. For the powdered specimens the sample holder was spun at 15 rpm. Phase identification was accomplished using a search-match software utilizing ICDD database (International Centre for Diffraction Data, Newtown Square, PA, USA).

Characterization of set materials

The set sealers were characterized using a combination of scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) analysis and Fourier transform infrared (FT-IR) spectroscopy. The characterization was performed on freshly prepared materials and also on sealers that had been immersed in 7 mL of Hank’s balanced salt solution (HBSS; H6648, Sigma–Aldrich, St. Louis, MO, USA) at 37 °C for 28 days. In addition, the surface morphology of the set sealers after 28-day of immersion in HBSS was assessed by scanning electron microscopy.

Scanning electron microscopy and X-ray energy dispersive analysis

Cylindrical specimens (10 mm in diameter and 2 mm thick) were prepared for the set cements. They were divided into three groups. Group 1 was allowed to set for 24 h at 37 ± 1 °C. Group 2 and 3 were allowed to set for 24 h at 37 ± 1 °C after which they were immersed in HBSS for 28 days. Both group 1 and 2 were impregnated in resin (Epoxyfix, Struers GmbH, Ballerup, Denmark) under vacuum. The resin blocks (groups 1 and 2) were then ground using progressively finer diamond discs and pastes using an automatic polishing machine (Tegramin 20, Struers GmbH, Ballerup, Denmark). The specimens in Group 3 were not embedded in resin and were dried in a desiccator with silica gel. All specimens were mounted on aluminum stubs, carbon coated and viewed under the scanning electron microscope (SEM; Leo 1430 Oxford, Cambridge, UK). Scanning electron micrographs of the different material microstructural components at different magnifications in back-scatter electron mode for Groups 1 and 2 were captured and X-ray energy dispersive spectroscopy (EDS) of the different phases was carried out. Group 3 was viewed using secondary electron imaging.

X-ray diffraction analysis

The set sealers were crushed using a mortar and pestle immediately after setting and after 28 days of immersion in HBSS. X-ray diffraction analysis was performed using the same parameters set for the raw materials.

Fourier transform infra-red spectroscopy

The composition of set sealers was investigated immediately after setting and after 28 days of immersion in HBSS using Fourier transform infrared (FT-IR) spectroscopy. To obtain the FT-IR spectrums from powder compounds and from the mixed material immediately after setting or after 28 days in HBSS, 2–5 mg of each powder component or crushed mixed samples of the sealers was added to 100 mg potassium bromide and analyzed in the IR spectrophotometer (Shimadzu IRAffinity-1; Shimadzu Corp., Kyoto, Japan) using transmitted infrared spectroscopy.

pH and calcium ion release

Polyethylene tubes ( n = 10) measuring 10 mm in length and 1 mm of internal diameter were filled with freshly prepared sealers and immersed in 10 mL distilled water. The specimens were stored at 37 °C throughout the experiment. Distilled water (10 mL) served as negative control. pH and calcium ion release were performed after 1, 7, 14, 21 and 28 days. At each time point the elution was collected for testing and solutions were replaced by fresh distilled water. The pH was measured using a previously calibrated digital pH meter (Digimed, Santo Amaro, SP, Brazil). The calcium ion release was assessed using an atomic absorption spectrophotometer (H1170 Hilger & Watts; Rank Precision Industries Ltd. Analytical Division, London, UK). The concentration of calcium ions released from the materials was quantified using a calcium hollow cathode lamp (422.7-nm wavelength and 0.7-nm window) operated at 20 mA. The readings of calcium ion release were compared with a standard curve obtained from multiple dilutions of pure calcium in ultrapure water.

Statistical analysis

The data were evaluated using SPSS (Statistical Package for the Social Sciences) software (PASW Statistics 18; SPSS Inc., Chicago Illinois, USA). Parametric tests were performed as K-S tests on the results indicated that the data were normally distributed. Analysis of variance (ANOVA) with P = 0.05 and Tukey post-hoc test were used to perform multiple comparison tests.

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Nov 25, 2017 | Posted by in Dental Materials | Comments Off on Chemical characterization and bioactivity of epoxy resin and Portland cement-based sealers with niobium and zirconium oxide radiopacifiers
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