Fig. 2.1
Back-scatter scanning electron micrograph of polished surface of MTA powder (Reprinted with permission from Camilleri 2007 [21]. Copyright ©2007, John Wiley and Sons)
Fig. 2.2
Energy-dispersive spectroscopy of a typical MTA powder and semi-quantitative elemental composition
The phases present in MTA are determined using X-ray diffraction analysis. Using this method of analysis, un-hydrated MTA exhibits peaks for tricalcium silicate, dicalcium silicate and bismuth oxide. Using a copper Kα tube, each phase has a particular pattern as shown in Fig. 2.3, which can then be searched and matched with data derived from the International Centre of Diffraction Data (ICDD) bank. Bismuth oxide (ICDD: 27-0053) exhibits typical peaks at 25.757, 26.906, 27.386, 28.010 and 33.229°2θ. Tricalcium silicate (ICDD: 86-0402) exhibits peaks at 29.414, 32.193, 32.504, 32.623, 34.355 and 41.298°2θ. Dicalcium silicate, usually exhibits a peak at 32.7°2θ, however such a peak is difficult to discern, due to its superimposition with the peaks present in the tricalcium silicate phase. Tricalcium aluminate is also present in un-hydrated MTA but in minimal quantities. X-ray diffraction analysis of MTA thus eliminates the myth that MTA is composed mainly of oxides. Thus, the term ‘trioxide aggregate’ is essentially a misnomer.
Fig. 2.3
X-ray diffractogram of un-hydrated MTA showing the main phases present
Quantitative phase analysis can be performed by Rietveld refinement using an internal standard such as rutile (titanium dioxide) added to the un-hydrated MTA. The principle of Rietveld analysis is to compare the experimental pattern with a pattern simulated based on the presumed amounts, crystal parameters and equipment parameters of a mixture of known phases. Rietveld refinement enables the amounts of different phases in anhydrous cementitious materials to be determined to a high degree of precision [71]. Quantitative assessment of ProRoot MTA (Dentsply Tulsa Dental, Johnson City, TN, USA) and MTA Angelus (Angelus, Londrina, Brazil) is shown in Table 2.1. Both cements exhibit different quantities of tricalcium silicate and dicalcium silicate when compared to Portland cement. The difference is due to variations in the manufacturing of Portland cement used as a raw material for preparing MTA. Phase analysis of ProRoot MTA has been published by Camilleri in 2008 [18] and has been validated by other researchers using Rietveld X-ray diffraction analysis [7] (Table 2.1). The main difference between the two studies is the presence of tricalcium aluminate in ProRoot MTA in the latter study [7]. Lack of an aluminate phase was also evident in scanning electron microscopy of polished sections of ProRoot MTA powder [18]. The absence of tricalcium aluminate phase, low levels of anhydrite and absence of gypsum may infer that the cement component in ProRoot MTA may not be a commercial Portland cement manufactured in a kiln but a laboratory-made cement. The manufacturer of ProRoot MTA (Dentsply Tulsa Dental Specialties, Tulsa, OK, USA) in fact claims that ProRoot MTA does not utilise a commercial Portland cement but the raw materials are certified for purity by inductively coupled plasma (ICP) spectroscopy. In the same document, the manufacturer also disclaims the presence of phosphate, which was stated to be the main constituent of MTA in the original publication [78]. In that publication, MTA was purportedly reported to be composed of ‘calcium oxide and calcium phosphate. Further analysis demonstrated that the former appeared as discrete crystals and the latter as an amorphous structure with no apparent crystal growth but a granular appearance. The mean value of the prisms was 87 % calcium and 2.47 % silica, the remainder being oxygen. In areas of amorphous structure, there seemed to be 33 % calcium, 49 % phosphate, 2 % carbon, 3 % chloride, and 6 % silica’.
Table 2.1
Rietveld X-ray diffraction analysis of the phases present in un-hydrated Portland cement, ProRoot MTA and MTA Angelus
|
Phases identified
|
Material type in mass %
|
||
|---|---|---|---|
|
Portland cement
|
ProRoot MTAa
|
MTA Angelusb
|
|
|
Tricalcium silicate
|
74.7
|
53.1 (51.9)
|
66.1
|
|
Dicalcium silicate
|
7.4
|
22.5 (23.2)
|
8.4
|
|
Tricalcium aluminate
|
3.6
|
0.0 (3.8)
|
2.0
|
|
Gypsum
|
1.1
|
0.0
|
0.0
|
|
Hemihydrate
|
1.1
|
0.0
|
0.0
|
|
Anhydrite
|
2.7
|
1.5 (1.3)
|
0.0
|
|
Calcium carbonate
|
5.0
|
1.4
|
0.0
|
|
Calcium oxide
|
0.0
|
0.0
|
8.0
|
|
Bismuth oxide
|
0.0
|
21.6 (19.8)
|
14.0
|
Another calcium silicate-based cement, MTA Angelus, was found to contain tricalcium aluminate but no sulphate-containing phase (Table 2.1). The absence of the gypsum is claimed by the manufacturer to reduce the setting time of the material. In fact, MTA Angelus has been shown to set in less than 50 min [59], as opposed to ProRoot MTA which was reported to have a setting time of over 2 h [9, 10, 20, 25, 30, 78]. Furthermore, 8 % calcium oxide is present in MTA Angelus [17]. This calcium oxide is a result of a raw mix with poor combustibility and with an unstable thermal profile. ProRoot MTA exhibits a higher level of bismuth oxide when compared to MTA Angelus (Table 2.1). This accounts for the higher radiopacity of ProRoot MTA [15, 20].
2.3 MTA Fineness
The MTA patent [79, 80] specifies the brand of Portland cement used in the original MTA formulation (Colton Fast-Set brand: Blaine number in the range of 4,500–4,600 cm2/g). The Blaine number is a numerical value which is calculated using the Blaine fineness measuring equipment. This is the industrial standard for measuring cement fineness. The Blaine method is specified by both European (EN 196-6) [34] and American (ASTM C204) [3] standards. Both standards specify an air permeability method wherein a bed of cement of known density is prepared; the resistance to a flow of air passing through the cement is measured and the fineness of the cement is calculated. This method measures cement fineness compared to standard cement. A range of 4,500–4,600 cm2/g is considered a fine cement.
Other industrial methods for measuring cement fineness include the Lea and Nurse apparatus. This method is an absolute method for determining the fineness of a cement and is adopted by most cement manufacturers. The particle size distribution of MTA has been calculated using laser particle size analysis [14], optical methods which involve measuring particle sizes of cement on polished sections of cement powder [5, 30], flow particle image analyser [54, 55] and by using the BET (Brunauer–Emmett–Teller) gas adsorption method [12] to calculate the specific surface area of MTA [13, 17]. The latter method is an extension of the Langmuir theory, which is a theory for monolayer molecular adsorption to multilayers. This theory hypothesises that gas molecules physically adsorb on a solid in layers infinitely; as there is no interaction between each adsorption layer, the Langmuir theory may be applied to each layer. The BET method is widely used in materials and surface science for the calculation of surface areas of solids by physical adsorption of gas molecules. By application of the BET theory, it is even possible to determine the inner surface area of hardened cement paste.
Optical assessment of MTA shows that the cement contains more uniform and finer-sized particles than Portland cement [5, 30]. However, other researchers using the same method reported coarser and more irregularly shaped crystalline particles in MTA [5]. Optical assessment shows that MTA consists of particles with diameters ranging from less than 1 μm to approximately 30 μm, and occasionally up to 50 μm. Particles of bismuth oxide (10–30 μm) are numerous [30]. Flow particle image analyses of various calcium silicate-based hydraulic cements indicate that the cumulative percentages of particles ranging from 6 to 10 μm for grey ProRoot MTA, white ProRoot MTA, grey MTA Angelus, white MTA Angelus and Portland cement are 65, 73, 48, 53 and 70 %, respectively. Thus, MTA Angelus contains a larger number of small particles with relatively low circularity and a wider range of size distribution and is less homogeneous than ProRoot MTA. Furthermore, white MTA contains smaller particles with a narrower range of size distribution than grey MTA [54]. The cumulative percentage of particles that are between 0.5 and 3 μm in size was reported to be 88 % [55]. Laser granulometry of MTA confirmed that this cement has a smaller particle size when compared to Portland cement [28]. Moreover, bismuth oxide, which is not present in Portland cement, exhibits a large particle size when examined microscopically [13] and with the use of laser granulometry [15].
ProRoot MTA and MTA Angelus were found to have a similar fineness when tested using the BET gas adsorption method [13, 17]. Both materials exhibit a specific surface area of approximately 1 m2/g. A novel MTA (MTA Plus compounded by Prevest Denpro, Jammu, India, for Avalon Biomed Inc. Bradenton, FL, USA) has a specific surface area of 1,537 m2/g [13], which is higher than the values obtained for MTA Angelus [17]. The higher specific surface area results in more surface available for cement reaction, which, in turn, results in a more rapid reaction rate.
2.4 Manipulation
Mineral trioxide aggregate is a water-based dental cement. It is usually supplied in pre-dosed powder and liquid that are mixed together to obtain a homogeneous paste. The recommended water/powder ratio is about 0.33. Changing the water/powder ratio affects the properties of MTA. The degree of solubility and the porosity of the cement increases when the water/powder ratio is increased. Cement pastes with a water/powder ratio higher than 0.33 are not viscous enough for clinical application. A ratio of 0.26 was the minimum that allowed a mix of putty consistency that can be manipulated [39]. Most MTA manufacturers supply prepacked 1 g powders with ampoules containing 0.33 g of water. Since the amount of material in each package is large enough for several applications, clinicians commonly estimate the amount of water and powder at the chairside, which results in using an unknown water/powder ratio. Variations in the water/powder ratio do not seem to affect the clinical performance of the material, No significant difference in material expansion [48] and no influence on the histological outcome was observed for MTA mixed at different water/powder ratios when used as a direct pulp-capping material on human healthy pulps [74]. When taking into consideration water/powder ratios in MTA, a distinction from water/cement ratio should be made. MTA contains 20 % bismuth oxide, which does not react with water. Thus effectively the water/cement ratio for MTA mixed at a water/powder ratio of 0.33 would be 0.41. Thus, comparison of properties of MTA with those of other systems using a different quantity or no radiopacifier or mineral additives is not possible since the effective water/cement ratio will vary depending on the quantity of additive. Modification of the water/cement ratio affects the properties of the set cement [29].
There have been a large number of reports on addition of various chemicals to the mixing liquid or replacement of the water by other liquids. The most popular is calcium chloride [1, 4, 10, 53, 56, 85], calcium nitrite/nitrate and calcium formate [85]. These chemical additions are also used in the industry to accelerate the setting of Portland cement. Setting accelerators affect the setting reaction of both tricalcium silicate and tricalcium aluminate [65].
Addition of water-soluble polymers [11, 14, 16, 19] increases material flow. ProRoot Endo Sealer is a commercial formulation using cement particles dispersed in a water-soluble polymer [49, 84]. The water-soluble polymer creates charges on the cement particles, resulting in repulsion of these charged particles with reduced flocculation and increased material flow at low water/cement ratios [65]. Propylene glycol has also been used to improve MTA flow [33]. Other polymers have been included to reduce washout of the unset cement. Anti-washout liquid is included in the MTA Plus formulation (compounded by Prevest Denpro, Jammu, India for Avalon Biomed Inc. Bradenton, FL, USA).
Other clinically available liquids have been added to MTA to improve its handling characteristics. These include local anaesthetic solution [40, 50, 53, 81, 83], sodium hypochlorite, chlorhexidine gluconate, saline and physiological solution [42, 50, 53], calcium lactate gluconate [51, 56] and citric acid [56]. The use of un-hydrated MTA as a root-end filling material has been reported [67]. Un-hydrated MTA will hydrate using the physiological fluid available at the root-end cavity. Non-specific and contradicting effects have been reported with the use of these chemicals. Physiological and synthetic tissue fluids contain chloride ions and glucose. The former is a cement hydration accelerator [65] while glucose is a hydration retarder. The combination of these effects may adversely alter the cement paste microstructure. Local anaesthetic solution contains both chloride and sulphate ions which again have a conflicting effect on cement hydration. A higher content of sulphate in the cement may lead to sulphate attack, whereby excessive expansion and cracking will be observed over time due to delayed ettringite deposition [64]. MTA is known to have low levels of sulphate ions and, although these ionic levels would alter the relative proportions of ettringite and monosulphate phases for a given degree of cement hydration, they are unlikely to create sulphate attack of the set cement. The chloride present in both synthetic tissue fluids and anaesthetic solutions may also alter the relative proportions of ettringite and monosulphate phases due to the formation of Friedel’s salt (calcium chloroaluminate, 3CaO•Al2O3•CaCl2•10H2O). The formation of Friedel’s salt will change the lattice structure of hydrated cement monosulphate phases and can potentially lead to microcracking [64].
Other variations to the mixing liquid include replacement of the water by various resins. These modifications result in the development of light-activated MTA and resin-modified MTA for use as root canal sealer cement. Resin-modified MTA reduces the setting time and enhances the bonding to both dentine and overlying composite, thus purportedly reducing micro-leakage. The use of a number of resin systems has been reported, with the main ones being light-curing systems containing bisphenol A-glycidyl methacrylate (bis-GMA) and a biocompatible resin [45, 46] consisting of 2-hydroxyethyl methacrylate (HEMA), triethylene glycol dimethacrylate (TEGDMA), camphorquinone and ethyl-4-(dimethylamino)benzoate (EDMAB), with or without polyacrylic co-maleic acid [42, 43], bis-GMA and TEGDMA [36] and bis-GMA, pyromellitic acid diethylmethacrylate (PMDM) and HEMA [70]. Chemically cured resins have also been employed [26, 36]. Other resins were added with the aim of increasing material flow, thus making MTA suitable to be used as a sealer cement [45, 46]. One such formulation is marketed by Angelus (Angelus, Londrina, Brazil) as MTA Fillapex. The latter is composed of MTA, a salicylate resin (methyl salicylate, butylene glycol and colophony) and other additives. Other proprietary brands such as MTA Obtura and Endo CPM Sealer exist. These sealer cements contain other additives to the MTA formula, that enhance material flow. Other experimental epoxy resin-based systems that incorporate MTA as fillers have also been reported as sealer cements [57].
Classically, MTA is mixed by manipulating the powder and liquid components on a mixing pad. Alternative mixing techniques such as the use of an amalgamator have been investigated [6, 63, 73]. MM MTATM, manufactured by MICRO-MEGA (Besançon Cedex, France), is supplied as MTA capsules that enable the MTA to be mixed using an amalgamator. Ultrasonic agitation has also been employed for mixing MTA [6, 63, 73]. The effectiveness of mechanical mixing and ultrasonic agitation is not clear, although mechanical mixing was shown to enhance the compressive strength of the set material, while ultrasonic agitation was found to improve the compressive strength of the material regardless of the mixing technique [6]. In addition to enhanced material micro-hardness [63], other research has shown that the various mixing methods have no significant effects on the resultant MTA mixtures [73]. Application of a condensation pressure of 1.68 MPa results in enhanced compressive strength. Higher condensation pressures result in fewer voids and microchannels, while specimens prepared with lower condensation pressures exhibit distinctive crystalline structures [62].
2.5 Washout
One of the drawbacks of MTA is washout after it is placed in situ. Washout refers to the tendency of a freshly prepared cement paste to ‘disintegrate upon early contact with blood or other fluids’ [82]. Washout can be measured using different methods. Most methods involve immersion of the unset cement in a liquid [24, 52, 58]. Agitation [82] or freeze drying [52] is then employed to disrupt the cement; alternatively the cement is sprayed with air from a specified distance [68]. The amount of material lost is then determined using photography [68] or quantified using a gravimetric method [52, 82]. One of the latest reported methods [37] includes a setup based on Specification CRD-C 661-06 [75] which was scaled down to allow testing of dental materials. When the results were compared to a metered water spray, they were found to yield quantitative, objective and reproducible results [37]. In the same study, MTA Plus and MTA Angelus exhibited washout when compared to Intermediate Restorative Material (IRM, Dentsply Caulk, Milford, Delaware, USA) and dental amalgam. The addition of anti-washout gel manufactured by Avalon Biomed Inc. reduced washout considerably [38].
2.6 Setting Reaction
Mineral trioxide aggregate hydrates when it comes in contact with water and undergoes two main reactions. The tricalcium silicate and dicalcium silicate react with water to form calcium silicate hydrate and calcium hydroxide. The tricalcium aluminate reacts with water and, in the presence of calcium sulphate, produces ettringite initially. When the sulphate-containing phases are depleted, a monosulphate phase is formed [21]:
(2.1)
(2.2)
(2.3)
The reactions that occur in MTA after hydration are the same reactions that are seen in Portland cement. During the initial stages of reaction, calcium silicate hydrate is formed; coating the cement particles with calcium silicate hydrate prevents further reaction. Tricalcium aluminate dissolves and reacts with the calcium and sulphate ions present in the liquid phase to produce ettringite that also precipitates on the cement particle surface. The initial phase is followed by a dormant period wherein the hydrate coating on the cement grains prevents further hydration. The dormant period lasts for 1–2 h, and is a period of relative inactivity when the cement is plastic and workable. Following the completion of the dormant period, setting of the cement proceeds to the acceleration stage wherein the hydration process accelerates again. The rate of tricalcium silicate hydration increases and more calcium silicate hydrate gel is formed. Hydration of dicalcium silicate also increases at this stage. Sulphate ions are depleted and monosulphate forms from ettringite. Crystalline calcium hydroxide also precipitates from the liquid phase.
The hydration progress can be monitored using calorimetry. The heat flux released by the chemical reaction is monitored over time and enables estimation of the beginning of setting and the rate of increase of the mechanical performance of the cement paste. Both MTA Angelus and MTA Plus present an initial endothermic peak followed by an exothermic peak (Fig. 2.4). The first 2 h correspond to an induction period followed by initial setting and then hardening of the material. The initial endothermic peak is due to the wetting of the surface. The first part of the exothermic peak is correlated with the very rapid and very exothermic hydration [13, 17]. It is interesting to note that MTA Angelus exhibits a higher exothermic peak, which occurs ahead of that of MTA Plus. This is caused by reaction of the calcium oxide present in MTA Angelus with water [17].
Fig. 2.4
Graphical representation of heat flux generated with time for MTA Plus and MTA Angelus
2.7 Characterisation of Set MTA
A combination of microscopy, elemental analysis and phase analysis has been used to characterise and evaluate the hydration mechanisms of MTA [13, 17, 18, 21]. The X-ray diffractograms are useful as MTA materials are mostly crystalline and individual mineral phases can be identified. Scanning electron microscopy allows observation of material microstructure and surface visualisation. Moreover, characterisation by X-ray energy dispersive analysis provides qualitative information of the elemental constitution of the test materials. Other useful methods have been employed for characterisation of MTA, including Fourier transform infrared spectroscopy (FT-IR) and Laser Raman spectroscopy. Laser Raman spectroscopy is largely complementary to infrared spectroscopy, but spectral interpretation is simpler [8]. The major components of Portland cement give distinctive Raman spectra [41, 44]. Both FT-IR and Raman spectroscopy have been used to investigate the interaction of calcium silicate cements with physiological solutions [27, 47, 76, 77]. These techniques are an adjunct to phase analysis by XRD and aid to verify the phases identified when peak overlap exists; which is the main disadvantage associated with the use of XRD to analyse Portland cement-based materials. This problem may be addressed by using the Rietveld method [71], which allows standardisation of powder diffraction analysis through the use of calculated reference diffraction patterns based upon crystal structure models.
Reaction by-products produced upon hydration of the cement are deposited around the periphery of the un-hydrated cement particles. As hydration proceeds, there is evidence of more reaction by-products. The hydration reaction takes several years to complete, although the cement mass would have achieved the final hardening and maximum physical and mechanical properties by 28 days. Hydrating MTA can be observed on back-scatter scanning electron micrographs of polished cement specimens (Fig. 2.5). The formation of cement by-products can be monitored by scanning electron microscopy in secondary electron mode. Calcium silicate hydrate exhibits a typical honeycomb appearance, while calcium hydroxide is deposited in the form of hexagonal plates (Fig. 2.6
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