Accepted Article

Received Date : 25-Mar-2014 Revised Date : 15-Aug-2014 Accepted Date : 20-Aug-2014 Article type

: Original Scientific Article

Investigation of a novel mechanically mixed Mineral Trioxide Aggregate (MM-MTA™)

I Khalil1, A Naaman1, J Camilleri2

1Department

of Endodontics, Faculty of Dentistry, St Joseph University, Beirut, Lebanon,

2Department

of Restorative Dentistry, Faculty of Dental Surgery, University of Malta, Malta

Key words MM-MTA, ProRoot MTA, MTA Angelus, characterization, physical properties, chemical properties

Running Head: Investigation of a mechanically mixed MTA

Correspondence: Dr Issam Khalil

This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process which may lead to differences between this version and the Version of Record. Please cite this article as an 'Accepted Article', doi: 10.1111/iej.12373 This article is protected by copyright. All rights reserved.

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Department of Endodontics, Faculty of Dentistry, St Joseph University, Beirut, Lebanon [email protected]

Aim To characterize a novel mechanically mixed mineral trioxide aggregate product (MM-MTA; MicroMega, Besançon, France) and to investigate the physical and chemical properties in comparison to ProRoot MTA (Dentsply, Tulsa Dental, Johnson City, TN, USA) and MTA Angelus (Angelus, Londrina, Brazil. Methodology The three materials were mixed according to manufacturer’s instructions. Specimens 10 mm in diameter and 2 mm high were prepared and characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analysis after 1 day and 28 days storage in physiological solution. Calcium ion leaching in solution and pH of the elution was also assessed. Furthermore the setting time, radiopacity and material porosity were investigated. Statistical analysis was performed by ANOVA and Tukey’s post hoc tests. Results All the MTAs tested were composed primarily of tricalcium silicate and bismuth oxide. In addition MM-MTA exhibited additional peaks for chlorine evident in the EDS analysis; calcium carbonate was present in the set material detected by XRD. Calcium hydroxide was present in the set ProRoot MTA and MTA Angelus. Calcium ion leaching and alkalization of the storage solution was demonstrated in all the materials. Both MM-MTA and MTA Angelus had a shorter setting time when compared to ProRoot MTA (P < 0.001). ProRoot MTA exhibited larger pores and more porosity than MTA Angelus and MM-MTA. All the materials exhibited radiopacity greater than the 3 mm aluminium thickness specified in ISO 6876 (2012). Conclusions MM-MTA, ProRoot MTA and MTA Angelus.are composed of Portland cement and bismuth oxide. In addition MM-MTA contains calcium carbonate and a chloride accelerator. These

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additives affect the material hydration and the properties of the set material. The properties of MMMTA are a result of a combination of factors namely the particular cement mineralogy, radiopacifier loading, effective water-cement ratio and mechanical mixing.

Introduction Mineral trioxide aggregate (MTA) is composed of Portland and bismuth oxide (Torabinejad & White 1995) and has two main disadvantages; its extended setting time (Torabinejad et al. 1995, Islam et al., 2006, Ber et al. 2007, Bortoluzzi et al. 2009) and difficulty in handling due to its sandy consistency (Johnson 1999). There are several reports of attempts at reduction in setting time of MTA with addition of calcium chloride being the easiest and most successful (Bortoluzzi et al. 2009, Lee et al. 2011). Biodentine (Septodont, Saint-Maur-des-Fossés, France) is a commercial material where the setting time of the tricalcium silicate is reduced by addition of a chloride additive (Camilleri et al. 2013a). The sandy consistency of MTA has been improved by addition of propylene glycol (Duarte et al. 2012) and water-soluble polymers (Camilleri et al. 2005, Camilleri 2008a); the latter are also employed in Biodentine.

Although other materials used in Dentistry offer several methods of mixing and delivery, MTA has been hand-mixed since its first use in 1995. Alternative mixing techniques such as the use of an amalgamator have been investigated (Basturk et al. 2013, Nekoofar et al. 2010, Shahi et al. 2012). Mechanical mixing was shown to enhance the compressive strength of the set material (Basturk et al. 2013). In addition to enhanced material micro-hardness (Nekoofar et al. 2010), other research has shown that the various mixing methods have no significant effects on the push-out bond strengths of the resultant MTA mixtures (Shahi et al. 2012).

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Recently a novel MTA, which is supplied in a capsule thus necessitates mechanical mixing has been developed (MM-MTATM, Micro-Mega, Besançon, France). The MM-MTA is pre-dosed and the activated capsule is mechanically mixed thus ensuring correct proportioning and homogenous mixing. Furthermore MTA contains additives that reduce the setting time of the material and enhances its properties (Micro-Mega webpage). The aim of this study was to characterize MM-MTA (Micro-Mega) and investigate the physical and chemical properties and compare these to ProRoot MTA (Dentsply, Tulsa Dental, Johnson City, TN, USA) and MTA Angelus (Angelus, Londrina, Brazil.

Methodology The materials used in this study included MM-MTA (Micro-Mega, Besançon, France), MTA Angelus (Angelus, Londrina, Brazil) and ProRoot MTA (Dentsply, Tulsa Dental, Johnson City, TN, USA). 
The ProRoot and MTA Angelus were manually mixed on a mixing pad using a water to powder ratio of 0.30. MM-MTA was mixed according to manufacturer’s instructions; the activated capsules were mixed in an amalgamator for 30 seconds following which the material was dispensed using a dispenser (Ketac Aplicap disperser, 3M Espe, St Paul, Minnesota, MN, USA). After mixing the materials were dispensed into moulds and compacted using an amalgam plugger. The material surface was smoothened with a glass slide.

Scanning electron microscopy and energy dispersive spectroscopy Specimens 10 mm in diameter and 2 mm thick were prepared. They were immersed in Hank’s balanced salt solution (HBSS; H6648, Sigma Aldrich, St. Louis, MO, USA) for 1 or 28 days at 37°C. The specimens were then vacuum desiccated and embedded in resin (Epoxyfix, Struers GmbH,

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Ballerup, Denmark) and ground using progressively finer diamond discs and pastes using an automatic polishing machine (Tegramin 20, Struers GmbH, Ballerup, Denmark). The polished specimens were attached to aluminium stubs, carbon coated and viewed under the scanning electron microscope (SEM; MERLIN Field Emission SEM, Carl Zeiss NTS GmbH, Oberkochen, Germany). Scanning electron micrographs of the microstructural components of the materials at different magnifications in back-scatter electron mode were captured. Energy dispersive spectroscopy (EDS) was carried out over the whole area being imaged.

X-ray Diffraction (XRD) analysis Phase analysis was carried out using X-ray diffraction. The materials aged for 1 day and 28 days in HBSS were dried in a vacuum desiccator and crushed to a 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 15-45°, with a sampling width of 0.05° and scan speed of 1°/min at 15 revs/min. Phase identification was accomplished using a search-match software utilizing ICDD database (International Centre for Diffraction Data, Newtown Square, PA, USA).

Assessment of pH and calcium ion leaching in solution Cylindrical specimens 10 mm in diameter and 2 mm in height were prepared, weighed and stored in 5 mL HBSS at 37°C for 1 and 28 days. The storage solution was tested for calcium ions using ion chromatography. Calcium ion release was calculated taking into consideration the sample size and volume of solution used. A blank HBSS solution was also analyzed in order to calculate the calcium ion content in the soaking solution. pH assessment of the leachate was also carried out after 1 and 28 days using a pH/mV/ISE meter (Hanna HI 3221, Hanna Instruments, Woonsocket, RI, USA) with a single-junction (Ag/AgCl) ceramic pH electrode (Hanna HI 1131, Hanna Instruments, Woonsocket, RI, USA). Temperature compensation was accomplished by simultaneously immersing

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a temperature probe (HI 7662, Hanna Instruments, Woonsocket, RI, USA) in the solution being tested.

Assessment of surface porosity Cylindrical specimens 10 mm in diameter and 2 mm high were prepared from each material type. They were allowed to harden for 1 day at 37°C and 100% humidity after which they were immersed in alcohol to stop the hydration, desiccated and vacuum impregnated in resin (Epoxyfix, Struers GmbH, Ballerup, Denmark). The resin blocks were then ground with progressively finer diamond discs and pastes using an automatic polishing machine (Tegramin 20, Struers GmbH, Ballerup, Denmark). The material surfaces were viewed under the light microscope (Axio Scope A1, Carl Zeiss, Göttingen, Germany) at 5 X and 10 X magnification to assess the surface porosity.

Radiopacity Radiopacity evaluation of the set materials was performed using ISO 6876:2012 recommendations. Three specimens 10 ± 1 mm in diameter and 1 ± 0.1 mm thick were used. 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., Wembley, UK) was used to irradiate X-rays onto the specimens using an exposure time of 1.60 seconds 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 (Adobe Photoshop) and a graph of thickness of aluminium vs. grey pixel value on the radiograph was plotted with the best-fit logarithmic trend line. The equation of the trend line gave the grey pixel value of an object on the image as a function

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of the object’s thickness in mm of aluminium. The grey pixel values of the cement specimens were then determined and the relevant thickness of aluminium calculated.

Assessment of setting time Setting time was evaluated using the procedure set out in ISO 9917-1:2007. The cement pastes were mixed and compacted into stainless steel moulds with internal cross-section 10 mm by 8 mm and depth 5 mm. A stopwatch was started and the moulds were immediately placed in an incubator at 37 ± 1°C immersed in gelatinized Hank’s balanced salt solution (HBSS; H6648, Sigma Aldrich, St. Louis, MO, USA). The gelatinized HBSS was prepared by mixing 10% porcine gelatin to HBSS heating until the gelatin dissolved. Testing for setting was done using a modified Vicat apparatus, consisting of a weighted needle of square cross-section of side 1 ± 0.01mm with a total mass of 400 ± 5 g. The cement was considered to have set when the needle was lowered gently onto the cement surface and did not leave a complete square indentation. The cement was tested for setting initially at 15 minutes time intervals, gradually shortening to around 1 minute intervals as time progressed and the cement was visibly close to being completely set.

Statistical Analysis The data were evaluated using SPSS (Statistical Package for the Social Sciences) software (PASW Statistics 18; SPSS Inc., Chicago, Ill, 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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Results Scanning electron microscopy and energy dispersive spectroscopy The scanning electron micrographs of the materials aged for 1 and 28 days in HBSS are shown in Figure 1 while the EDS analysis of the materials in Figure 2. All the three materials exhibited hydration product deposited around the cement particles and in the cement matrix. The hydration product was evident even in the first day and was more accentuated at 28 days of hydration. Bright shiny particles composed of bismuth and oxygen were identified in the cement matrix. The EDS analysis showed similar elemental distribution for all the materials with major peaks for calcium, silicon, bismuth, aluminium and oxygen. The presence of aluminium indicates that the materials are based on Portland cement as opposed to pure tricalcium silicate, which lacks the aluminate phase. MTA Angelus lacked the sulfur and MM-MTA included peaks for chlorine.

X-ray Diffraction (XRD) analysis Assessment of hydrated materials was also performed using XRD analysis (Figure 3). All the materials exhibited peaks for tricalcium silicate at 29.504, 32.187, 32.501, 34.256° 2θ and bismuth oxide at 26.923, 27.378, 33.040° 2θ. The bismuth oxide peak in MM-MTA was of lower intensity than that presented for MTA Angelus and ProRoot MTA. MTA Angelus and ProRoot MTA exhibited a peak for calcium hydroxide (Portlandite) at 18.004 and 34.098°2θ both after 1 day and 28 days of cement hydration. The peak was higher for MTA Angelus at both time frames and for both materials the peak intensified with cement aging. MM-MTA did not exhibit the Portlandite peak at either time periods. MM-MTA exhibited an additional peak of calcium carbonate (calcite) at 29.459°2θ.

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Assessment of pH and calcium ion leaching in solution The calcium ion leaching in Hank’s balanced salt solution after 1 day and 28 days of specimen storage is shown in Figure 4a. The leaching of calcium ions increased over the 28-day period with MM-MTA exhibiting the highest leaching at both time intervals. ProRoot MTA exhibited high calcium ion leaching at 1 day, which was maintained over the 28-day period. The results of pH of the storage solution are shown in Figure 4b. All storage solutions exhibited an alkalizing pH at all time periods.

Assessment of material porosity The light micrographs at different magnifications for the test materials are shown in Figure 5. Although this test method is qualitative, ProRoot MTA exhibited larger pores and more porosity than MTA Angelus and MM-MTA. The MM-MTA was the least porous material tested. Surface porosity was negligible. Radiopacity The material radiopacity is shown in Figure 6. All the materials exhibited radiopacity greater than the 3 mm aluminium thickness specified in ISO 6876 (2012). MM-MTA exhibited the lowest radiopacity followed by MTA Angelus and ProRoot MTA. The difference in material radiopacity was significant (p < 0.002). Assessment of setting time The setting time of the materials is shown in Figure 7. MM-MTA exhibited a comparable setting time to MTA Angelus (p = 0.5). Both MM-MTA and MTA Angelus demonstrated a shorter setting time when compared to ProRoot MTA (p > 0.001).

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Discussion The materials investigated in this study included three commercially available mineral trioxide aggregates manufactured by different companies. The ProRoot MTA and MTA Angelus were mixed by hand at the same powder/liquid ratio while the MM-MTA was provided in capsules, which were activated and mixed in an automatic mixer. The materials were compacted into moulds using amalgam pluggers without any condensation pressure. The application of pressure on the fresh cement has been shown to affect the surface micro-hardness and compressive strength of MTA (Nekoofar et al. 2007). These tests were not undertaken in the current study thus the use of condensation pressure was not considered relevant.

The microstructure of hydrated MTAs was investigated using a combination of scanning electron microscopy and energy dispersive spectroscopy of polished specimens in combination to phase analysis by X-ray diffractometry. These techniques have been utilized for the assessment of hydration of MTA (Camilleri 2007, 2008b, Camilleri et al. 2013b) and related materials (Camilleri et al. 2013a). MTA Angelus and ProRoot MTA are both composed of a mixture of Portland cement and bismuth oxide. The ProRoot MTA was shown to contain ~20% bismuth oxide (Camilleri 2008, Belio Reyes et al. 2009) and as suggested by the manufacturer as opposed to the lower amounts reported for MTA Angelus (Camilleri et al. 2013a). The lower amount of bismuth oxide in MTA Angelus reflects in the lower radiopacity of the material. The radiopacities of the ProRoot MTA and MTA Angelus are in accordance to previous research (Torabinejad et al. 1995, Tanomaru-Filho et al. 2008, Vivan et al. 2009, Camilleri & Gandolfi 2010). MM-MTA exhibited lower radiopacity than both ProRoot MTA and MTA Angelus. The bismuth oxide peak on the X ray diffractogram in MM-MTA was lower when compared to that present in the other MTAs tested indicating a lower level of bismuth oxide in MM-MTA.

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MM-MTA is presented in capsule form thus it facilitates the mixing procedure when compared to both ProRoot MTA and MTA Angelus. All the materials revealed the presence of calcium, silicon and aluminium on EDS analysis as Portland cement is the main cementitious phase rather than pure tricalcium silicate. MTA Angelus did not exhibit a sulfur peak on EDS analysis indicating the absence of a sulfate phase in the material. Calcium sulfate is added to Portland cement by the manufacturer, in order to retard the setting time of the material used for construction purposes. The reduction of the calcium sulfate from the cement will inadvertently block the formation of ettringite during hydration and allow the calcium aluminate to flash set and form calcium aluminate hydrate with a reduction in material setting time (Taylor 1997).

MTA Angelus was also shown in previous research to contain calcium oxide, which was postulated to be present due to poor sintering of the cement during manufacture. The calcium oxide contained in MTA Angelus increases the hydration rate and results in an exothermic reaction shown by calorimetric measurements. Furthermore the presence of calcium oxide increases the quantity of calcium hydroxide produced as a by-product of reaction (Camilleri et al. 2013a). The increased Portlandite peak in MTA Angelus was also shown in the current study.

MM-MTA was composed of tricalcium silicate, bismuth oxide and calcium carbonate as indicated by the XRD analysis. The EDS analysis exhibited the presence of chlorine. The calcium carbonate and chloride were not present in the other MTAs tested. The addition of chlorides to Portland cement systems is also routine occurrence in the construction industry. Chlorides are used as accelerators thus reducing the setting time of the material; the most frequently employed is calcium chloride. The effect of calcium chloride on MTA has been reported (Bortoluzzi et al. 2009, Lee et al. 2011). Calcium carbonate is also added to Portland cement used in construction as a filler. When added to

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Portland cement, calcium carbonate was shown to accelerate tricalcium silicate hydration thus reducing the setting time of the materials (Ramachandran 1988, Pera et al. 1999). This is in accordance to the claims by MM-MTA manufacturer (Micro-Mega web page) that the calcium carbonate is added to reduce the setting time.

Calcium carbonate and chloride accelerator are also present in Biodentine, which is a related material composed of tricalcium silicate, zirconium oxide and calcium carbonate (Camilleri et al. 2013a). The calcium carbonate in Biodentine acts a nucleation site for calcium silicate hydrate thus reducing the duration of the induction period. As a consequence of a short induction period, the initial setting time of the material is expected to be less; this was demonstrated for Biodentine (Camilleri et al. 2013a). MM-MTA was also shown to have a lower setting time than Pro Root MTA (Torabinejad et al. 1995) and similar to MTA Angelus (Bortoluzzi et al. 2009) verified in the current study. The shorter setting time of MM-MTA is very likely a result of the combination of effect of addition of calcium carbonate and a chloride accelerator. The effect of addition of calcium carbonate to a Portland cement system is different than that of tricalcium silicate cement. Biodentine does not contain an aluminate phase and does not form ettringite on hydration. In Portland cements the addition of calcium carbonate affects the formation of ettringite during hydration thus modifying the hydration process. During the formation of ettringite, the sulfate ions are replaced by carbonate ions producing calcium carbosilicate and calcium carboaluminate. This modifies the cement hydration mechanisms (Bushnell-Watson & Sharp 1985). MTA Angelus and Pro Root MTA exhibited the formation of calcium hydroxide in the hydrated phase both at 1 day and 28 days. The calcium hydroxide is formed as a by-product of tricalcium silicate hydration. Although MM-MTA contained tricalcium silicate as the main cementitious phase, no Portlandite peak was detected on X-ray diffraction analysis. On hydration calcium ions are

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leached in solution. The presence of calcium in solution was demonstrated in the assessment of leachates by ion chromatography. MM-MTA had higher levels of calcium ions in solution regardless the absence of Portlandite peak on XRD analysis. The presence of calcium ions in solution could be caused by decomposition of calcium carbonate thus releasing calcium in solution. The lack of detection of Portlandite in the XRD scans is postulated to be resulting from the calcium carbonate in the MM-MTA powder. Calcium carbonate has been shown to impede crystallization of Portlandite. Non-crystalline Portlandite cannot be detected by X-ray diffraction analysis since XRD detects only crystalline phases (Hawkins et al. 2003, Gabrovšek et al. 2006). Calcium carbonate is reactive and affects the distribution of lime, alumina and sulfate and thereby alters the mineralogy of hydrated cement pastes. Calcite additions affect the amount of free calcium hydroxide as well as the balance between ettringite and monosulfate phases in the hydrated cement phases (Matschei et al. 2007). The storage solutions in contact with the MTAs exhibited an alkaline pH.

Material porosity can be determined visually from observation of pore size and distribution of polished materials. Porosity can also be determined quantitatively by working out the material porosity from sorption and solubility data (ISO 4049) and also by mercury intrusion porosimetry where mercury is intruded in material pores. Mercury intrusion porosimetry (MIP) has proven to be a valid and reliable technique for evaluating pore size distribution and total open porosity of endodontic materials (Saghiri et al. 2012, Camilleri et al. 2014). In the present study the surface porosity was assessed from images of polished sections using the light microscope as in previous literature (Cutajar et al. 2011). This method is not as precise as MIP as only the pores present in that specific area which may not be representative of the whole material can be assessed. Furthermore this technique is not quantitative.

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Porosity is a common characteristic of cements and occurs as a result of the spaces between the unhydrated cement grains. As the hydration reaction progresses, the hydration products fill these gaps and reduce the porosity. Material porosity is dependent on the water-cement ratio. If a high water-cement ratio is used during mixing, excess water eventually dries off and leaves voids that are not filled by hydration products. Porosity is observed to increase with an increase in water to cement ratio (Raymond & Kenneth 1991) and decreases as the cement ages. Three varieties of pores can be distinguished: closed, through and blind pores (Webb & Orr 1997). Closed pores are inaccessible to fluids; blind pores terminate inside the material, whilst through pores facilitate the complete passageway of fluids. “Open porosity” includes only through and blind pores. Closed pores have a significant effect on the mechanical properties of the cement, and the open porosity directly impacts the ease with which undesirable oral fluids can penetrate into unprotected dentine (Milutinovid-Nikolid et al. 2007).

The method used in this study to investigate material porosity has been previously reported together with the assessment of porosity from sorption/solubility data calculated by ISO 4049; 2009 (Cutajar et al. 2011). In the latter study the porosity was assessed after 28 days of storage as opposed to the current study, which evaluated material porosity immediately after setting. ProRoot MTA and MTA Angelus were mixed at the same water-powder ratio. ProRoot MTA exhibited higher porosity than MTA Angelus. Although mixed at the same water-powder ratio the MTAs have a different radiopacifier loading. ProRoot MTA includes 20% bismuth oxide (Camilleri 2008b) while MTA Angelus contains 14% (Camilleri et al. 2013a). Thus, the effective water-cement ratio of ProRoot MTA is higher. Varying the radiopacifier loading requires modification of water-powder ratio in order to keep the water-cement ratio stable thus avoiding variations in physical properties of the materials (Cutajar et al. 2011). The water-powder ratio of MM-MTA was not known as the

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material is supplied with a pre-dosed capsule. MM-MTA exhibited low porosity which could be a related to a low water-powder ratio and the effect of the mechanical mixing. The setting time of cements was assessed with the materials immersed in a physiological solution to mimic the clinical scenario. MTA is used in contact with tissue fluid when employed as a root-end filling material and to repair root perforations. The HBSS was gelatinized to avoid material washout. Washout is the tendency of freshly prepared cement paste to disintegrate upon contact with fluids. MTA has been shown to be susceptible to washout (Formosa et al. 2013).

The setting time of MTA was shown to be longer when the material is in contact with simulated body fluids (Formosa et al. 2012, Camilleri et al. 2013). Both ProRoot MTA and MTA Angelus failed to set when in contact with fetal bovine serum (Kim et al. 2012). The retardation of cement hydration in contact with simulated body fluids is induced by the formation of insoluble hydroxides in alkaline solution. The insoluble hydroxides form a coating over the cement particles. The adsorption of phosphate ions at the surface of the clinker phase or on the first hydration product is thought to result in the precipitation of calcium phosphates (Ramachandran 1995). Furthermore the setting time results are dependent on the technique of evaluation. The weight of the indenter and the size of the mould affect the results of setting time. Camilleri 2010 used a similar technique to that employed in the current study thus data can be compared between the two studies and the differences in the result obtained attributed to the use of a physiological solution in the current study as opposed to testing at 100% humidity in the previous assessment (Camilleri 2010). Other researchers (Islam et al. 2006, Santos et al. 2008, Bortoluzzi et al. 2009, Grazziotin-Soares et al. 2014) used ASTM C266-08 (2008). The ASTM C266-08 method determines both initial and final setting times using Gilmore needles with a different head weight. The main disadvantage however is the need to use 650 g of cement to fill a mould of dimensions 100 by 5 mm. The mould

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dimensions were adapted to 10 by 2 mm (Ber et al. 2007, Bortoluzzi et al. 2009, Grazziotin-Soares et al. 2014) to take up less material. These mould are too shallow when compared to the size suggested by ISO 9917 (2007) thus giving results that are not comparable to studies using standard moulds.

ProRoot MTA exhibited a longer setting time compared to previous reports (Islam et al. 2006, Ber et al. 2007, Camilleri 2010). In a recent study the setting time of ProRoot MTA was assessed in the presence of blood and media and ProRoot MTA set in 36 hours (Charland et al. 2013). The setting time for MTA Angelus was also longer than most of the previous data published for this material. Previous reports tested the material stored at 100% humidity (Santos et al. 2008, Bortoluzzi et al. 2009, Vivan et al. 2010). A recent study reported a setting time of 165 minutes for MTA Angelus (Grazziotin-Soares et al. 2014), which is much higher than previous reports using the same methodology and storage at 100% humidity. This setting time was also higher than the results of the current study where a soaking solution was used throughout the testing.

Conclusions MM-MTA is composed primarily of Portland cement and bismuth oxide like ProRoot MTA and MTA Angelus. In addition it contains calcium carbonate and a chloride accelerator. These additives affect the material hydration and the properties of the set material. The MM-MTA properties are a result of a combination of factors namely the particular cement mineralogy, radiopacifier loading, effective water-cement ratio and the mechanical mixing.

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References American Standards for Testing Materials ASTM C-266 (2008) Standard Test Method for Time of Setting of Hydraulic-Cement Paste by Gillmore Needles. Basturk FB, Nekoofar MH, Günday M, Dummer PM (2013) The effect of various mixing and placement techniques on the compressive strength of mineral trioxide aggregate. Journal of Endodontics 39, 111-4. Belío-Reyes IA, Bucio L, Cruz-Chavez E (2009) Phase composition of ProRoot mineral trioxide aggregate by X-ray powder diffraction. Journal of Endodontics 35, 875-8. Ber BS, Hatton JF, Stewart GP (2007) Chemical modification of ProRoot MTA to improve handling characteristics and decrease setting time. Journal of Endodontics 33, 1231-4. Bortoluzzi EA, Broon NJ, Bramante CM, Felippe WT, Tanomaru Filho M, Esberard RM (2009) The influence of calcium chloride on the setting time, solubility, disintegration, and pH of mineral trioxide aggregate and white Portland cement with a radiopacifier. Journal of Endodontics 35,550-4. Bushnell-Watson SM, Sharp JM (1985) The detection of the carboaluminate phase in hydrated high alumina cements by differential thermal analysis. Thermochimica Acta 93, 613–6. Camilleri J (2007) Hydration mechanisms of mineral trioxide aggregate. International Endodontic Journal 40, 462-70. Camilleri J (2008a) The physical properties of accelerated Portland cement for endodontic use. International Endodontic Journal 41, 151-7. Camilleri J (2008b) Characterization of hydration products of mineral trioxide aggregate. International Endodontic Journal 41, 408-17.

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Camilleri J (2010) Evaluation of the physical properties of an endodontic Portland cement incorporating alternative radiopacifiers used as root-end filling material. International Endodontic Journal 43, 231- 40.

Camilleri J, Formosa L, Damidot D (2013b) The setting characteristics of MTA Plus in different environmental conditions. International Endododontic Journal 46, 831- 40. Camilleri J, Gandolfi MG (2010) Evaluation of the radiopacity of calcium silicate cements containing different radiopacifiers. International Endodontic Journal 43, 21-30. Camilleri J, Grech L, Galea K, Keir D, Fenech M, Damidot D, Mallia B (2014) Assessment of porosity and sealing ability of tricalcium silicate-based root-end filling materials. Clinical Oral Investigation 18, 1437-46. Camilleri J, Montesin FE, Di Silvio L, Pitt Ford TR (2005) The constitution and biocompatibility of accelerated Portland cement for endodontic use. International Endodontic Journal 38, 834-42. Camilleri J, Sorrentino F, Damidot D (2013a) Investigation of the hydration and bioactivity of radiopacified tricalcium silicate cement, Biodentine and MTA Angelus. Dental Materials 29, 580-93. Charland T, Hartwell GR, Hirschberg C, Patel R (2013) An evaluation of setting time of mineral trioxide aggregate and EndoSequence root repair material in the presence of human blood and minimal essential media. Journal of Endodontics 39, 1071-2. Cutajar A, Mallia B, Abela S, Camilleri J (2011) Replacement of radiopacifier in mineral trioxide aggregate; characterization and determination of physical properties. Dental Materials 27, 879-91.

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Duarte MA, Alves de Aguiar K, Zeferino MA, Vivan RR, Ordinola-Zapata R, Tanomaru-Filho M, Weckwerth PH, Kuga MC (2012) Evaluation of the propylene glycol association on some physical and chemical properties of mineral trioxide aggregate. International Endodontic Journal 45, 565-70. Formosa LM, Mallia B, Camilleri J (2013) A quantitative method for determining the antiwashout characteristics of cement-based dental materials including mineral trioxide aggregate. International Endodontic Journal 46, 179-86. Formosa LM, Mallia B, Camilleri J (2012) The effect of curing conditions on the physical properties of tricalcium silicate cement for use as a dental biomaterial. International Endodontic Journal 45, 326-36. Gabrovšek R, Vuk T, Kaučič V (2006) Evaluation of the hydration of Portland cement containing various carbonates by means of thermal analysis. Acta Chimica Slovenia 53, 159-65. Grazziotin-Soares R, Nekoofar MH, Davies TE, Bafail A, Alhaddar E, Hübler R, Busato AL, Dummer PM (2014) Effect of bismuth oxide on white mineral trioxide aggregate: chemical characterization and physical properties. International Endodontic Journal 7, 520-33 Hawkins P, Tennis PD, Detwiler R (2003) The Use of Limestone in Portland Cement: A State-of-theArt Review, EB 227, Portland Cement Association, Skokie, 44. International Standards Organization (ISO 4049: 2009) Dentistry–Polymer-based restorative materials. International Standards Organization (ISO 9917-1: 2007) Dentistry-Water-based cements Part 1: Powder/liquid acid-base cements. International Standards Organization (ISO 6876: 2012) Dentistry-Root canal sealing materials.

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Figure legends Figure1 Back-scatter scanning electron micrographs of the cements stored in HBSS for 1 and 28 days. Figure 2 Energy dispersive analysis of the different MTAs Figure 3 X-ray diffraction plots of the materials stored in HBSS for 1 and 28 days Figure 4 a: Calcium ion leaching after 1 and 28 day immersion of test materials in Hank’s balanced salt solution. b: pH of leachate after 1 and 28 day immersion of test materials in Hank’s balanced salt solution

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Figure 5 Light micrographs of polished materials showing surface porosity (mag 5x, 10X) Figure 6 Radiopacity of materials expressed in thickness of aluminium (± SD) Figure 7 Setting time of the three variants of MTA (± SD)

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Accepted Article This article is protected by copyright. All rights reserved.

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Investigation of a novel mechanically mixed mineral trioxide aggregate (MM-MTA(™) ).

To characterize a novel mechanically mixed mineral trioxide aggregate product (MM-MTA, MicroMega, Besançon, France) and to investigate the physical an...
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