Calcium silicate-based sealers: Assessment of physicochemical properties, porosity and hydration

Highlights

  • Calcium silicate-based sealer using water and propylene as vehycle were assessed.

  • All prototype sealers prodeuced clacium hydroxide during hydration and favored alkaline pH and calcium release.

  • Zirconium oxide and calcium tungstate provided adequate radiopacity which was dose related.

Abstract

Objectives

Investigation of hydration, chemical, physical properties and porosity of experimental calcium silicate-based sealers.

Methods

Experimental calcium silicate-based sealers with calcium tungstate and zirconium oxide radio-opacifiers were prepared by mixing 1 g of powder to 0.3 mL of 80% distilled water and 20% propylene glycol. MTA and MTA Fillapex were used as controls. The raw materials and set sealers were characterized using a combination of scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. Physical properties were analyzed according to ANSI/ADA. The pH and calcium ion release were assessed after 3, 24, 72 and 168 h. The porosity was assessed using mercury intrusion porosimetry.

Results

The analysis of hydration of prototype sealers revealed calcium hydroxide as a by-product resulting in alkaline pH and detection of calcium ion release, with high values in initial periods. The radiopacity was similar to MTA for the sealers containing high amounts of radio-opacifiers ( p > 0.05). Flowability was higher and film thickness was lower for resinous MTA Fillapex sealer ( p < 0.05). The test sealers showed water sorption and porosity similar to MTA ( p > 0.05).

Significance

The prototype sealers presented adequate hydration, elevated pH and calcium ion release. Regarding physical properties, elevated proportions of radio-opacifiers were necessary to accomplish adequate radiopacity, enhance flowability and reduce film thickness. All the tested sealers presented water sorption and porosity similar to MTA.

Introduction

Mineral trioxide aggregate (MTA) is a calcium silicate-based cement mainly composed of Portland cement, with the addition of bismuth oxide as radio-opacifier . MTA is used for apexifications, apical surgeries, pulp capping and for root perforations . This cement is widely known by its bioactive properties that include stimulation of tissue repair and induction of mineralization . These properties are essential, mainly because of the direct contact with the pulpal and periodontal tissues.

The favorable biological characteristics have led to the development of root canal sealers based on tricalcium silicate . The handling characteristics of MTA preclude its use as a sealer without the addition of chemicals that provide sufficient flow. The manufacturers of MTA recommend mixing with distilled water, which results in a sandy and dry material . Substances including water-soluble polymer and a gel have been tested in order to establish their efficacy in enhancing the cement manipulation. Propylene glycol (PG) is frequently used in Dentistry as a vehicle for calcium hydroxide and has also been tested as an additive to improve MTA mixing .

MTA Fillapex is a paste-catalyst MTA-containing sealer (Angelus Soluções Odontológicas, Londrina, PR, Brazil). The paste A is composed of salicylate resin (methyl salicylate, butylene glycol, colophony), bismuth oxide and fumed silica. The paste B includes fumed silicon dioxide, titanium dioxide, base resin (pentaerythritol, rosinate, p – toluenesulfonamide) and MTA. MTA Fillapex contains 40% of MTA in its composition. The studies show that this sealer is biocompatible, stimulates mineralization and exhibits bioactivity by stimulating hydroxyapatite nucleation .

The use of alternative radio-opacifiers to bismuth oxide in MTA has also been tested . Reports that bismuth oxide interferes with MTA hydration and causes negative effects in the mechanical properties have encouraged research on the substitution of bismuth oxide by other radio-opacifiers. Furthermore root canal sealers containing bismuth oxide should be avoided since traces of sodium hypochlorite used as root canal irrigant may remain inside the root canal. Sodium hypchlorite reacts with bismuth oxide resulting in a black precipitate . This reaction results in tooth discoloration since bismuth migrates from the MTA to the adjacent dentin . Zirconium oxide has been tested as a substitute to bismuth oxide due to its adequate radiopacity and no interference with hydration of Portland cement . Calcium tungstate has also been investigated as alternative radio-opacifier to bismuth oxide, and it shows adequate physical and chemical properties when associated with Portland cement .

The aim of the study was to test the physical properties, porosity, hydration, pH and calcium ion release of prototype calcium silicate-based sealers in comparison with the MTA and MTA Fillapex.

Materials and methods

The prototype sealers were composed of Portland cement (PC, Irajazinho, Cimento Rio Branco, Rio de Janeiro, Brazil), 20% or 50% zirconium oxide (PC-20-Zr; PC-50-Zr, Sigma–Aldrich, St. Louis, MO, USA) or 20% or 50% calcium tungstate (PC-20-CT; PC-50-CT, Sigma–Aldrich). The prototype sealers were mixed at a powder to liquid ratio of 0.3. The liquid was composed of 80% distilled water and 20% propylene glycol (C 3 H 8 O 2 ). The propylene glycol was added to enhance the material properties and enabling the cements to be used as root canal sealers .

The MTA (Angelus, Londrina, Paraná, Brazil) and MTA Fillapex (Angelus) were the controls. MTA was mixed with distilled water at a powder to liquid ratio of 0.3. MTA Fillapex is a paste-catalyst sealer and was mixed with the same proportion of each paste.

Characterization of materials

Both un-hydrated cement powders and the set materials were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analysis.

Microscopy and elemental analysis

The powders were prepared by mixing the powder with epoxy resin (Epoxyfix, Struers GmbH, Ballerup, Denmark). For the hydrated materials cylindrical specimens 10 mm in diameter and 2 mm high were prepared and stored in Hank’s Balanced Salt Solution (HBSS, H6648, Sigma–Aldrich) for 28 days at 37 °C. At the end of the storage period the specimens were removed form the solution and dried in a vacuum desiccator. They were then embedded in resin (Epoxyfix, Struers GmbH, Ballerup, Denmark). Both embedded powders and set materials were ground and polished using progressively finer diamond discs and pastes using an automatic polishing machine (Tegramin 20, Struers GmbH, Ballerup, Denmark). The polished specimens were attached to aluminum stubs, carbon coated and viewed under the scanning electron microscope (SEM; Zeiss MERLIN Field Emission SEM, Carl Zeiss NTS GmbH, Oberkochen, Germany). Scanning electron micrographs of the different material microstructural components at different magnifications in back-scatter electron mode were captured and energy dispersive spectroscopy (EDS) was carried out.

X-ray diffraction (XRD) analysis

Phase analysis was carried out using X-ray diffraction. After storage in HBSS for 28 days the set materials were dried in a vacuum desiccator and crushed to a very fine powder using an agate mortar and pestle. The diffractometer (Rigaku, Tokyo, Japan) used Cu Kα radiation at 40 mA and 45 kV and the detector was set to rotate between 15° and 45°, with a sampling width of 0.05° and scan speed of 1°/min at 15 revs/min using the Bragg Brentano method. Phase identification was accomplished using a search-match software utilizing ICDD database (International Centre for Diffraction Data, Newtown Square, PA, USA).

pH and calcium ion release in solution

Eighty acrylic teeth ( n = 10) made of resin with a cavity of 3-mm depth were filled with the different materials and immersed individually in 10 mL of deionized water and stored at 37 °C. To avoid any interference with the results, all glass flasks were pre-treated with nitric acid. After 3, 24, 72 and 168 h, the teeth were placed in new flasks containing an equal volume of new deionized water. The pH of the water in which the teeth had been kept was measured with a pH meter (model 371; Micronal, São Paulo, SP, Brazil), previously calibrated using buffer solutions of pH 4, 7 and 14. After the removal of the specimens, the container was placed in a shaker (model 251; Farmem, São Paulo, SP, Brazil) for 5 s before measuring. The temperature of the room during the reading was 25 °C.

For determination of calcium ion release, an atomic absorption spectrophotometer (AA6800; Schimadzu, Tokyo, Japan) equipped with a calcium- specific hollow cathode lamp was used. The water in which calcium ion release was measured was the same used in the pH measurements. To prevent potential interferences from alkaline metals, a solution of lanthanum was prepared by diluting 9.8 g of lanthanum nitrate in 250 mL of acid solution. A stock solution of calcium was prepared by diluting 2.4972 g of calcium carbonate in 50 mL of ultrapure water. A total of 10 mL of concentrated hydrochloric acid was added to this solution. Subsequently, it was diluted with 1000 mL of ultrapure water. After the preparation, 1 mL of this solution corresponded to 1 mg of calcium. With this solution, standard solutions containing 10, 20, 40 and 80 mg L −1 of calcium were prepared. Then, 2 mL of lanthanum nitrate solution was added to 6 mL of the standard calcium or test solutions. For the preparation of the blank solution, the same amount of lanthanum nitrate solution was added to 6 mL of ultrapure water. The calcium standards, the blank and the test solutions were read in the atomic absorption spectrophotometer. To bring the device to zero absorbance, a solution of nitric acid was used. The readings of the calcium ion release were compared with a standard curve obtained from readings of the standard solutions. This reading was performed in the same periods used for the pH level measurement.

Radiopacity

Three cylindrical samples of each sealer with 10 mm in diameter and 1 mm in thickness were prepared. The thickness was checked with a digital caliper (Mitutoyo Corp, Tokyo, Japan) and the samples radiographed on occlusal films (D-speed; Kodak Comp, Rochester, NY) with an aluminum step-wedge graduated from 2 to 16 mm. A radiographic unit (Gnatus XR 6010; Gnatus, Ribeirão Preto, SP, Brazil) was used with exposures set at 60 kVp, 10 mA, 0.3 s and a focus-film distance of 30 cm. The radiographs were digitized and analyzed using Digora 1.51 software (Soredex, Helsinki, Finland). The radiopacity was determined as previously described .

Flowability

A volume of 0.5 mL of sealer was placed on a glass plate according to ANSI/ADA’s specification No. 57/2000 . Three minutes after the start of mixing, another plate with a mass of 20 ± 2 g and a load of 100 g plus was applied centrally on top of the plate. Ten minutes after the start of mixing, the load was removed, and the average of the major and minor diameters of the compressed sealer was measured using a digital caliper (Mitutoyo MTI Corporation, Tokyo, Japan). Three measurements were performed for each sealer.

Film thickness

Two 5-mm-thick flat glass plates were joined and their thickness measured. A total of 0.5 mL of sealer was placed on the center of one plate and the second was positioned centrally on top of the sealer, according to ANSI/ADA No. 57/2000 . Three minutes after the start of mixing, a load of 150 N was applied vertically on top of the glass plate. Ten minutes after the start of mixing, the thickness of the two glass plates and the interposed sealer was measured with a digital caliper (Mitutoyo MTI Corporation). The difference of three measurements for each sealer in thickness of the two glass plates, with and without sealer, was taken as the film thickness of the material.

Solubility

Three 1.5-mm-thick cylindrical polytetrafluoroethylene molds with an inner diameter of 20 mm were filled with freshly mixed sealer. The mold was supported by a larger glass plate and covered with a cellophane sheet. A nylon thread was placed inside the material, and another glass plate, also covered with cellophane film in such a way that the plates touched the entire mold in a uniform manner. The assembly was placed in an incubator (37 °C, 95% relative humidity) for a period corresponding to three times the setting time. The sealers were removed from the mold and weighed three times each with an accuracy of 0.0001 g (UMark 210, Bel Engineering, Monza, Italy). The samples were suspended by nylon thread and placed two by two inside a plastic vessel containing 50 mL of deionized distilled water. The containers were stored for 24 h in an incubator (37 °C, 95% relative humidity). Then, the samples were rinsed with deionized distilled water, blotted dry with absorbent paper, placed in desiccators for 24 h, and then reweighed. The experiment was repeated three times for each sealer. The percentage of weight loss of each sample (initial mass minus final mass) was considered the solubility of the sealer, according to ANSI/ADA specification No. 57/2000 .

Assessment of material porosity

The porosity of the set sealers was assessed after 28-day hydration in Hank’s Balanced Salt Solution (HBSS; H6648, Sigma Aldrich, St. Louis, MO, USA). The method of mercury intrusion porosimetry was used to measure the average pore diameter and percentage of porosity. Three cube specimens 7 mm × 7 mm × 7 mm were prepared for each material, allowed to set for 24 h at 37 °C and subsequently immersed in at 37 °C for 1 or 28 days. The materials were taken out of solution and dried for 4 days in an incubator at 60 °C. The porosity was measured in a two-stage process using a mercury intrusion porosimeter (PoreMaster, Quantachrome Instruments, New York, NY, USA). Calibrated mercury displacement pycnometry followed by low and high-pressure mercury intrusion porosimetry were performed. The volume of mercury intruded in the pores of the specimen was measured. Porosimetry data was processed using the software supplied with the machine (Poremaster, Quantachrome Instruments, New York, NY, USA).

A gas pycnometer (Quantachrome Instruments, New York, NY, USA) with helium was used to determine the average absolute density (g/cm 3 ). The pycnometer operates on Archimedes principle of gas displacement to determine the volume. Density of the sample can be calculated from the sample weight and volume.

Statistical analysis

The statistical analysis for porosity was performed using Kruskal–Wallis and Dunn’s tests and for physical tests, pH and calcium ion release, one-way ANOVA and Tukey’s tests as a result of normal distribution confirmed by Kolmogorov–Smirnov test ( p < 0.05). The data were evaluated using SPSS (Statistical Package for the Social Sciences) software (PASW Statistics 18; SPSS Inc., Chicago, IL, USA).

Materials and methods

The prototype sealers were composed of Portland cement (PC, Irajazinho, Cimento Rio Branco, Rio de Janeiro, Brazil), 20% or 50% zirconium oxide (PC-20-Zr; PC-50-Zr, Sigma–Aldrich, St. Louis, MO, USA) or 20% or 50% calcium tungstate (PC-20-CT; PC-50-CT, Sigma–Aldrich). The prototype sealers were mixed at a powder to liquid ratio of 0.3. The liquid was composed of 80% distilled water and 20% propylene glycol (C 3 H 8 O 2 ). The propylene glycol was added to enhance the material properties and enabling the cements to be used as root canal sealers .

The MTA (Angelus, Londrina, Paraná, Brazil) and MTA Fillapex (Angelus) were the controls. MTA was mixed with distilled water at a powder to liquid ratio of 0.3. MTA Fillapex is a paste-catalyst sealer and was mixed with the same proportion of each paste.

Characterization of materials

Both un-hydrated cement powders and the set materials were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analysis.

Microscopy and elemental analysis

The powders were prepared by mixing the powder with epoxy resin (Epoxyfix, Struers GmbH, Ballerup, Denmark). For the hydrated materials cylindrical specimens 10 mm in diameter and 2 mm high were prepared and stored in Hank’s Balanced Salt Solution (HBSS, H6648, Sigma–Aldrich) for 28 days at 37 °C. At the end of the storage period the specimens were removed form the solution and dried in a vacuum desiccator. They were then embedded in resin (Epoxyfix, Struers GmbH, Ballerup, Denmark). Both embedded powders and set materials were ground and polished using progressively finer diamond discs and pastes using an automatic polishing machine (Tegramin 20, Struers GmbH, Ballerup, Denmark). The polished specimens were attached to aluminum stubs, carbon coated and viewed under the scanning electron microscope (SEM; Zeiss MERLIN Field Emission SEM, Carl Zeiss NTS GmbH, Oberkochen, Germany). Scanning electron micrographs of the different material microstructural components at different magnifications in back-scatter electron mode were captured and energy dispersive spectroscopy (EDS) was carried out.

X-ray diffraction (XRD) analysis

Phase analysis was carried out using X-ray diffraction. After storage in HBSS for 28 days the set materials were dried in a vacuum desiccator and crushed to a very fine powder using an agate mortar and pestle. The diffractometer (Rigaku, Tokyo, Japan) used Cu Kα radiation at 40 mA and 45 kV and the detector was set to rotate between 15° and 45°, with a sampling width of 0.05° and scan speed of 1°/min at 15 revs/min using the Bragg Brentano method. Phase identification was accomplished using a search-match software utilizing ICDD database (International Centre for Diffraction Data, Newtown Square, PA, USA).

pH and calcium ion release in solution

Eighty acrylic teeth ( n = 10) made of resin with a cavity of 3-mm depth were filled with the different materials and immersed individually in 10 mL of deionized water and stored at 37 °C. To avoid any interference with the results, all glass flasks were pre-treated with nitric acid. After 3, 24, 72 and 168 h, the teeth were placed in new flasks containing an equal volume of new deionized water. The pH of the water in which the teeth had been kept was measured with a pH meter (model 371; Micronal, São Paulo, SP, Brazil), previously calibrated using buffer solutions of pH 4, 7 and 14. After the removal of the specimens, the container was placed in a shaker (model 251; Farmem, São Paulo, SP, Brazil) for 5 s before measuring. The temperature of the room during the reading was 25 °C.

For determination of calcium ion release, an atomic absorption spectrophotometer (AA6800; Schimadzu, Tokyo, Japan) equipped with a calcium- specific hollow cathode lamp was used. The water in which calcium ion release was measured was the same used in the pH measurements. To prevent potential interferences from alkaline metals, a solution of lanthanum was prepared by diluting 9.8 g of lanthanum nitrate in 250 mL of acid solution. A stock solution of calcium was prepared by diluting 2.4972 g of calcium carbonate in 50 mL of ultrapure water. A total of 10 mL of concentrated hydrochloric acid was added to this solution. Subsequently, it was diluted with 1000 mL of ultrapure water. After the preparation, 1 mL of this solution corresponded to 1 mg of calcium. With this solution, standard solutions containing 10, 20, 40 and 80 mg L −1 of calcium were prepared. Then, 2 mL of lanthanum nitrate solution was added to 6 mL of the standard calcium or test solutions. For the preparation of the blank solution, the same amount of lanthanum nitrate solution was added to 6 mL of ultrapure water. The calcium standards, the blank and the test solutions were read in the atomic absorption spectrophotometer. To bring the device to zero absorbance, a solution of nitric acid was used. The readings of the calcium ion release were compared with a standard curve obtained from readings of the standard solutions. This reading was performed in the same periods used for the pH level measurement.

Radiopacity

Three cylindrical samples of each sealer with 10 mm in diameter and 1 mm in thickness were prepared. The thickness was checked with a digital caliper (Mitutoyo Corp, Tokyo, Japan) and the samples radiographed on occlusal films (D-speed; Kodak Comp, Rochester, NY) with an aluminum step-wedge graduated from 2 to 16 mm. A radiographic unit (Gnatus XR 6010; Gnatus, Ribeirão Preto, SP, Brazil) was used with exposures set at 60 kVp, 10 mA, 0.3 s and a focus-film distance of 30 cm. The radiographs were digitized and analyzed using Digora 1.51 software (Soredex, Helsinki, Finland). The radiopacity was determined as previously described .

Flowability

A volume of 0.5 mL of sealer was placed on a glass plate according to ANSI/ADA’s specification No. 57/2000 . Three minutes after the start of mixing, another plate with a mass of 20 ± 2 g and a load of 100 g plus was applied centrally on top of the plate. Ten minutes after the start of mixing, the load was removed, and the average of the major and minor diameters of the compressed sealer was measured using a digital caliper (Mitutoyo MTI Corporation, Tokyo, Japan). Three measurements were performed for each sealer.

Film thickness

Two 5-mm-thick flat glass plates were joined and their thickness measured. A total of 0.5 mL of sealer was placed on the center of one plate and the second was positioned centrally on top of the sealer, according to ANSI/ADA No. 57/2000 . Three minutes after the start of mixing, a load of 150 N was applied vertically on top of the glass plate. Ten minutes after the start of mixing, the thickness of the two glass plates and the interposed sealer was measured with a digital caliper (Mitutoyo MTI Corporation). The difference of three measurements for each sealer in thickness of the two glass plates, with and without sealer, was taken as the film thickness of the material.

Solubility

Three 1.5-mm-thick cylindrical polytetrafluoroethylene molds with an inner diameter of 20 mm were filled with freshly mixed sealer. The mold was supported by a larger glass plate and covered with a cellophane sheet. A nylon thread was placed inside the material, and another glass plate, also covered with cellophane film in such a way that the plates touched the entire mold in a uniform manner. The assembly was placed in an incubator (37 °C, 95% relative humidity) for a period corresponding to three times the setting time. The sealers were removed from the mold and weighed three times each with an accuracy of 0.0001 g (UMark 210, Bel Engineering, Monza, Italy). The samples were suspended by nylon thread and placed two by two inside a plastic vessel containing 50 mL of deionized distilled water. The containers were stored for 24 h in an incubator (37 °C, 95% relative humidity). Then, the samples were rinsed with deionized distilled water, blotted dry with absorbent paper, placed in desiccators for 24 h, and then reweighed. The experiment was repeated three times for each sealer. The percentage of weight loss of each sample (initial mass minus final mass) was considered the solubility of the sealer, according to ANSI/ADA specification No. 57/2000 .

Assessment of material porosity

The porosity of the set sealers was assessed after 28-day hydration in Hank’s Balanced Salt Solution (HBSS; H6648, Sigma Aldrich, St. Louis, MO, USA). The method of mercury intrusion porosimetry was used to measure the average pore diameter and percentage of porosity. Three cube specimens 7 mm × 7 mm × 7 mm were prepared for each material, allowed to set for 24 h at 37 °C and subsequently immersed in at 37 °C for 1 or 28 days. The materials were taken out of solution and dried for 4 days in an incubator at 60 °C. The porosity was measured in a two-stage process using a mercury intrusion porosimeter (PoreMaster, Quantachrome Instruments, New York, NY, USA). Calibrated mercury displacement pycnometry followed by low and high-pressure mercury intrusion porosimetry were performed. The volume of mercury intruded in the pores of the specimen was measured. Porosimetry data was processed using the software supplied with the machine (Poremaster, Quantachrome Instruments, New York, NY, USA).

A gas pycnometer (Quantachrome Instruments, New York, NY, USA) with helium was used to determine the average absolute density (g/cm 3 ). The pycnometer operates on Archimedes principle of gas displacement to determine the volume. Density of the sample can be calculated from the sample weight and volume.

Statistical analysis

The statistical analysis for porosity was performed using Kruskal–Wallis and Dunn’s tests and for physical tests, pH and calcium ion release, one-way ANOVA and Tukey’s tests as a result of normal distribution confirmed by Kolmogorov–Smirnov test ( p < 0.05). The data were evaluated using SPSS (Statistical Package for the Social Sciences) software (PASW Statistics 18; SPSS Inc., Chicago, IL, USA).

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Nov 23, 2017 | Posted by in Dental Materials | Comments Off on Calcium silicate-based sealers: Assessment of physicochemical properties, porosity and hydration
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