“MTA”—An Hydraulic Silicate Cement: Review update and setting reaction

Abstract

Objectives

To review the current status and understanding of Portland cement-like endodontic materials commonly referred to by the trade designation “MTA” (alias “Mineral Trioxide Aggregate”), and to present an outline setting reaction scheme, hitherto unattempted.

Method

The literature was searched using on-line tools, overlapping an earlier substantial review to pick up any omissions, including that in respect of ordinary Portland cement (OPC), with which MTA shares much. The search was conducted for the period January 2005 to December 2009 using ‘MTA’, ‘GMTA’, ‘WMTA’, and ‘mineral AND trioxide AND aggregate’ as keywords, with various on-line search engines including ScienceDirect ( ), SAGE Journals Online ( ), Wiley Online Library ( ), SciELO Scientific electronic library online ( www.scielo.br/scielo.php ), JSTOR ( ), and Scopus ( ). References of articles found were cross-checked where appropriate for missed publications. Manufacturers’ and related websites were searched with Google Search ( www.google.com.hk ).

Results

A generic name for this class of materials, Hydraulic Silicate Cement (HSC), is proposed, and an outline reaction scheme has been deduced. HSC has distinct advantages apparent, including sealing, sterilizing, mineralizing, dentinogenic and osteogenic capacities, which research continues to demonstrate. However, ad hoc modifications have little supporting justification.

Significance

While HSC has a definite place in dentistry, with few of the drawbacks associated with other materials, some improvements in handling and other properties are highly desirable, as are studies of the mechanisms of the several beneficial physiological effects. Reference to the extensive, but complex, literature on OPC may provide the necessary insight.

Introduction

The cementitious material generally known by its trade name of “Mineral Trioxide Aggregate” (MTA) has rapidly gained acceptance in dentistry since its introduction in 1993 by Torabinejad . Patented and marketed in 1995 , it was given approval for endodontic applications in 1998 . Camilleri and Pitt Ford reviewed the constituents and biocompatibility of MTA based on literature published from November 1993 to August 2005, while Roberts et al. have given a comprehensive review of reports on MTA materials based on an electronic search of the literature from January 1990 to August 2006, and it is not proposed to go over that ground again. However, despite this body of work over a long period, and apart from a few relatively superficial remarks, the setting reaction has not been given a clear or satisfactory explanation. The purpose now then is to extend that review to date, and to present an accessible account of the probable setting reactions with a view to stimulating research on the topic and enable its teaching. The review covers primarily the literature from January 2005 to December 2009, overlapping Roberts et al. slightly to pick up articles that might have been missed, but also includes some cited then to draw out specific points. Recently, Parirokh and Torabinejad have presented a substantial and useful key to the literature, but while comprehensive this does not set out to offer critical comment. The goal here is more interpretative. Accordingly, the references given are selective, for key articles, rather than exhaustive.

The scientific dental literature was searched, using ‘MTA’, ‘GMTA’, ‘WMTA’, and ‘mineral AND trioxide AND aggregate’ as keywords, with various on-line search engines including ScienceDirect ( ), SAGE Journals Online ( ), Wiley Online Library ( ), SciELO Scientific electronic library online ( www.scielo.br/scielo.php ), JSTOR ( ), and Scopus ( ). References of articles found were cross-checked where appropriate for missed publications. Manufacturers’ and related websites were searched with Google Search ( www.google.com.hk ).

General description

MTA is available in several forms ( Table 1 ), presented as a very fine powder which is to be mixed with sterile or distilled water; both components are provided pre-dosed for a water–powder mass ratio of 0.35 mL/g for ProRoot MTA, and 0.28 mL/g for MTA Angelus, to be mixed on a sterilized glass slab for 30–60 s to obtain a “sandy” consistency. According to the manufacturers, the working time is ∼5 min (although this is said to lengthened by covering the mixed material with moistened gauze to slow down evaporation, even though its setting is not by drying). ProRoot MTA is said to have a setting time of 4 h, while the Angelus product, which has less calcium sulfate, the setting (hardening) time is, according to the manufacturer, said to be decreased from 2.5 h to 15 min (but then inappropriate, and not advised by the manufacturer, for canal obturation).

Table 1
Details of some commercial brands of Hydraulic Silicate Cement. Data from the manufacturer unless otherwise referenced.
Product Manufacturer Composition Remarks Other properties
Gray ProRoot MTA Dentsply Tulsa Dental, Tulsa, Oklahoma, USA O (30.5), Ca (37.2), Si (7.9), S (0.8), Mg (1.0), Al (1.7), K (0.3), Fe (2.8), Bi (17.9) (mass%) Powder–water ratio: 1/0.35 g/mL sterile water in ampoule; setting time: 4 h; working time: ∼5 min; hydration of powder creates colloidal gel, fully cured in 4 weeks ‘tooth-colored’ replaced former White ProRoot MTA 6–10 fraction a : 65% compressive strength: 28.4 MPa @ 7 d 67 MPa @ 7 d
Tooth-colored ProRoot MTA Reduced Fe content 6–10 fraction: 73% HV50: 69 ± 7 .
Gray MTA Angelus Angelus Indústria de Produtos Odontológicos Ltda., Londrina, PR, Brazil SiO 2 , Na 2 O, K 2 O, Al 2 O 3 , Fe 2 O 3 , SO 3, Bi 2 O 3 , CaO, MgO, insoluble residues of CaO, K 2 SO 4 , Na 2 SO 4 and crystalline silica Powder–water ratio: 0.14/0.04 g/mL; initial setting time: 5 min
final setting time: 15 min
6–10 fraction: 48%
White MTA Angelus Reduced Fe 2 O 3 content 6–10 fraction: 53% compressive strength: 44.2 MPa @ 28 d
CPM Egeo S.R.L., Buenos Aires, Argentina SiO 2 , K 2 O, Al 2 O 3 , SO 3 , CaO, Bi 2 O 3 , CaCO 3 , BaSO 4 White
developed: 2004
setting time: 1 h
added CaCO 3 “to enhance Ca ion release and control alkalinity”

a Proportion of powder particles lying in size range 6–10 μm .

General description

MTA is available in several forms ( Table 1 ), presented as a very fine powder which is to be mixed with sterile or distilled water; both components are provided pre-dosed for a water–powder mass ratio of 0.35 mL/g for ProRoot MTA, and 0.28 mL/g for MTA Angelus, to be mixed on a sterilized glass slab for 30–60 s to obtain a “sandy” consistency. According to the manufacturers, the working time is ∼5 min (although this is said to lengthened by covering the mixed material with moistened gauze to slow down evaporation, even though its setting is not by drying). ProRoot MTA is said to have a setting time of 4 h, while the Angelus product, which has less calcium sulfate, the setting (hardening) time is, according to the manufacturer, said to be decreased from 2.5 h to 15 min (but then inappropriate, and not advised by the manufacturer, for canal obturation).

Table 1
Details of some commercial brands of Hydraulic Silicate Cement. Data from the manufacturer unless otherwise referenced.
Product Manufacturer Composition Remarks Other properties
Gray ProRoot MTA Dentsply Tulsa Dental, Tulsa, Oklahoma, USA O (30.5), Ca (37.2), Si (7.9), S (0.8), Mg (1.0), Al (1.7), K (0.3), Fe (2.8), Bi (17.9) (mass%) Powder–water ratio: 1/0.35 g/mL sterile water in ampoule; setting time: 4 h; working time: ∼5 min; hydration of powder creates colloidal gel, fully cured in 4 weeks ‘tooth-colored’ replaced former White ProRoot MTA 6–10 fraction a : 65% compressive strength: 28.4 MPa @ 7 d 67 MPa @ 7 d
Tooth-colored ProRoot MTA Reduced Fe content 6–10 fraction: 73% HV50: 69 ± 7 .
Gray MTA Angelus Angelus Indústria de Produtos Odontológicos Ltda., Londrina, PR, Brazil SiO 2 , Na 2 O, K 2 O, Al 2 O 3 , Fe 2 O 3 , SO 3, Bi 2 O 3 , CaO, MgO, insoluble residues of CaO, K 2 SO 4 , Na 2 SO 4 and crystalline silica Powder–water ratio: 0.14/0.04 g/mL; initial setting time: 5 min
final setting time: 15 min
6–10 fraction: 48%
White MTA Angelus Reduced Fe 2 O 3 content 6–10 fraction: 53% compressive strength: 44.2 MPa @ 28 d
CPM Egeo S.R.L., Buenos Aires, Argentina SiO 2 , K 2 O, Al 2 O 3 , SO 3 , CaO, Bi 2 O 3 , CaCO 3 , BaSO 4 White
developed: 2004
setting time: 1 h
added CaCO 3 “to enhance Ca ion release and control alkalinity”

a Proportion of powder particles lying in size range 6–10 μm .

Applications

MTA has been reported to be an osteogenic, biocompatible material, inductive and conductive of hard tissue formation . By virtue of its strong alkalinity it is bactericidal and stimulates cementum-like hard tissue formation , osteoblastic adherence , and bone regeneration . It has found many applications: direct pulp capping , pulpotomy , root-end filling , apexification and apexogenesis in immature pulpal necrotic teeth, endodontic obturation of the root canal , treatment of horizontal root fracture , repair of resorptive defects – including both internal and external root resorption , and repair of perforations at root canal and furcation levels. Current approaches for indirect pulp therapy using this material, direct pulp capping, and pulpotomy for both primary teeth and permanent teeth with open apices, have been comprehensively reviewed by Camp and Fuks .

Pulpotomy dressing agent

Pulpotomy is a vital pulp therapy indicated for deciduous teeth with exposed pulp and reversible pulpitis. MTA is believed to preserve pulp vitality by disinfecting, sealing and inducing the formation of a calcific barrier over the radicular pulp tissue (again, presumably due to its high pH). This is in contrast with the superficial fixation ( i.e. in the histological sense) of healthy radicular tissue with formocresol in conventional pulpotomy. Clinical studies over 24–42 months have indicated a high success rate for pulpotomy using MTA (67–98.5%), comparable with formocresol (77–83%) and ferric sulfate (73%), but not with calcium hydroxide (46%) . The apparent contradiction here may be due to the relatively rapid dissolution of the latter while the high pH over, and thus cytological activity of, MTA is maintained . The incidence of post-operative internal root resorption was lowest for MTA . In contrast to MTA, and in addition to concerns arising from the toxicity and genotoxicity of formocresol , both formocresol and ferric sulfate were found to irritate and cause comparatively substantial inflammatory responses of the pulp in animal studies . Although MTA can induce initial inflammation of rat dental pulp cells via its effect on the nuclear factor-kappa B signaling system , a subsequent anti-inflammatory effect due to down-regulation of the expression of certain inflammatory mediators is also observed . Such chemical effects are in need of detailed explanation as the implications for treatment and material design could be of importance.

Necrotic immature permanent teeth

Sealing of necrotic immature permanent teeth with open apices using MTA is considered to be a viable treatment alternative to the conventional use of both Ca(OH) 2 -assisted apexification and resin-, zinc oxide-, glass ionomer- or amalgam-based root-end endodontic surgical retrograde filling of open apical foramina , accompanied by canal obturation with gutta percha. This is the case whether the MTA is used in orthograde filling over sterile radicular blood clot (revascularization) or residual vital apical radicular tissue (apexogenesis) ; in the direct formation of a plug as an artificial apical barrier (apexification) or complete orthograde obturation ; or even in simple retrograde root-end filling .

Direct pulp-capping agent

Combined histological and immunohistochemical analyses in human and animal studies have demonstrated reparative dentin formation by odontoblast-like cells. These have originated from the differentiation of progenitors which proliferated and pooled at the site of MTA capping. Thus, MTA appears to work to preserve exposed pulp vitality in permanent teeth by promoting the formation of dentin bridges, although the otherwise-described disinfectant and sealant properties would be complementary. MTA-induced reparative dentinogenesis was found to be more consistent and prominent compared with direct pulp-capping with Ca(OH) 2 .

Chemistry

MTA is a so-called hydraulic cement ( i.e. sets and is stable under water ), relying primarily on hydration reactions for setting, as opposed to the more usual acid–base systems used in dentistry, but its solubility is relatively low . In broad terms, the material consists primarily of calcium silicates, with a variety of other phases present, depending on additions made to modify properties.

No generic term for this class of materials appears to have been adopted, reliance being made on the original trade name. Despite precedents for the continuation of such a label in other (non-dental) contexts, a generic name will become necessary when patents expire (2013-04-23) and other manufacturers of other versions emerge, but it is in any case desirable for teaching, research, insurance, trade and standardization purposes. The term “Hydraulic Silicate Cement” (HSC) is therefore proposed, or just “Hydraulic Silicate” (HS). The use of the qualifying ‘hydraulic’ is necessary and sufficient to distinguish such materials from the older dental silicate cements which relied on reaction with phosphoric acid ( i.e. an acid–base system).

The similarity of MTA to ordinary Portland cement (OPC) is not accidental, being both the inspiration 1

1 LK Bakland, pers. comm., Nov. 2009.

and preliminary trial material . Indeed, the original patent says “in a preferred embodiment, the principal component is Portland cement.” . However, OPC itself is unworkable in the dental context owing to concerns about its heavy metal constituents , lack of adequate radio-opacity , comparatively large setting expansion , broad distribution in particle size and relatively high solubility in some forms; nor does it have US federal approval for clinical purposes . The modifications that have been made in designing MTA, discussed below, have addressed concerns of particle size, setting rate, solubility and, possibly, toxicity (by the reduction of heavy metal content). However, OPC has been shown to have similar antimicrobial and sealing ability to MTA. Addition of bismuth oxide alone or together with calcium sulfate provides adequate radio-opacity to OPC , and the arsenic content is negligible in some commercial brands of OPC . Thus, MTA Angelus is based on a low-arsenic OPC .

The chemical similarity between MTA and OPC ( Table 2 ) has been demonstrated by comparative studies of their elemental ( Table 3 ) and phase compositions ( Tables 3 and 4 ) using energy-dispersive X-ray (EDX) analysis, X-ray photoelectron spectroscopy (XPS), inductively coupled plasma optical emission spectroscopy (ICP-OES), and X-ray diffraction (XRD) . With the exception of bismuth oxide (Bi 2 O 3 ) (more properly, bismuth sesquioxide or bismuth(III) oxide), which is a specific addition in MTA for radio-opacity, MTA and OPC have components in common. CaO (from limestone) makes up 50–75 mass%, while SiO 2 and Al 2 O 3 (both from clay or shale) constitute 15–25 mass% and 2–5 mass% respectively ( Table 2 ). MgO, Fe 2 O 3 , and trace amounts of other heavy metals are common ore impurities found in Gray MTA and OPC . Calcium sulfate is added to modify setting behavior in both MTA and OPC.

Table 2
Comparison of Gray MTA, White MTA with OPC .
Aspect MTA Ordinary Portland cement (OPC)
White MTA Gray MTA
CaO/mass% 50–75
SiO 2 /mass% 15–25
Al 2 O 3 /mass% <2 2–5
Fe 2 O 3 /mass% 0–0.5 2–5
Bi/at% ∼2 (∼20 mass%) Nil
Fe, Mn chromophores Negligible + +
Toxic heavy metals (Cu, Mn, Sr)/mass% Low Appreciable
Calcium sulfate a As anhydrite, hemihydrate, dihydrate
Particle size, distribution Uniform, smaller Uniform, larger Wide size range

a Sulfate (∼half that of OPC) reported as gypsum by some , but as anhydrite only by others .

Table 3
Comparison of analytical results (all mg/g), for White ProRoot MTA and OPC: X-ray photoelectron spectroscopy (XPS) and inductively-coupled plasma optical emission spectroscopy (ICP-OES) .
XPS ICP-OES
Element OPC White MTA Element OPC White MTA
Mg 0 0 Mg 4.17 2.19
Al 26 11 Al 12.08 6.59
S 28 8 S 27.98 19.88
Fe 20 0 Fe 10.46 1.49
K 26 0 K 5.83 0.69
Bi 0 149 Bi 0.00 136.45
Ca 316 333 B 0.07 0.00
Si 103 68 Ba 0.13 0.01
O 426 366 Cl 0.92 0.49
Cu 0.04 0.04
Ga 0.08 0.18
In 0.06 0.12
Li 0.19 0.04
Mn 0.16 0.00
Ni 0.00 0.08
Sr 1.74 0.70
Tl 0.01 0.02
Zn, Ag, Cd, Co, Cr, Pb 0.00 0.00

Table 4
Chemical composition of ordinary Portland cement (OPC). Constitution calculated from (phase key: see text “CCN” and Table 5 ).
Composition Constitution
Component Content/mass% Phase Content/mass%
SiO 2 19.7 C 3 S 55
CaO 63.2 C 2 S 15
CaO (free) 0.46 C 3 A 7.9
Al 2 O 3 4.7 C 4 AF 8.1
Fe 2 O 3 2.67 CaO 0.46
SrO 0.07 CaCO 3 4.4
K 2 O 1.12 CaSO 4 a 4.5
Na 2 O 0.08 K 2 SO 4 1.6
MgO 1.85 Na 2 SO 4 0.096
CO 2 1.93 SrO 0.07
SO 3 3.35

a As anhydrite (2.5), hemihydrate (0.5) and dihydrate (1.5).

It is worth noting that in OPC the concentration of sulfate is controlled to obtain optimal drying shrinkage, strength, plasticity and delay in setting . Gypsum is added during grinding to prevent so-called ‘flash set’ . This is consistent with the remark above about the setting time of the Angelus product.

For comparison, OPC has an average particle aspect ratio (length/diameter) of 0.7 and ∼70% of the particles lie in the size range of 6–10 μm, compared with 65, 73, 48 and 53% for Gray ProRoot MTA, White ProRoot MTA, Gray MTA Angelus and White MTA Angelus, respectively . There are, however, differences in shape and size distributions of the particles between the various versions: White MTA has smaller particles with a narrower size distribution than the Gray .

Tooth-colored (also called White) ProRoot MTA was introduced in about 2002, the main characteristic of which was the near-elimination (<0.5 mass%) from the original formulation of iron, depleting the set MTA of aluminoferrite, which was responsible for the gray coloration. This color was problematic in some circumstances where the cosmetic appearance of the treated tooth was affected adversely. For reasons which are not clear, the aluminum content was also much reduced , while the arsenic content was reduced , presumably on toxicity concerns.

The smaller particle size of White MTA means it has a greater specific surface area, which in turn causes an increase in the wetting volume, water-binding capacity and hydration rate . At the same water–powder ratio, White MTA will be thicker, which together with an increase in the cohesiveness, a better workability is expected in comparison with Gray MTA. However, it is reported that the so-called “hygroscopic” linear setting expansion of Gray MTA is greater than that of White MTA .

Manufacture

The manufacturing process of MTA is the same as that of OPC . In outline, the raw materials, limestone, shale or clay, and bauxite, are crushed and blended in the required proportions, and heated gradually in a rotary kiln to 1400–1500 °C. Free water is evaporated, decomposition (calcination) of CaCO 3 and clay follows, losing bound H 2 O and CO 2 , to produce the so-called clinker in the form of spherical nodules (1–25 mm) with the composition indicated in Table 3 . (The almost complete lack of ferrite and aluminate phases, which serve to lower the clinkering temperature, has prompted Camilleri to question whether laboratory production has taken over clinkering in a rotary kiln in the manufacture of tooth-colored or White MTA ). After cooling, calcium sulfate is added and the mixture ground to a very fine powder . In MTA, sulfate is reported to be present in the form of gypsum, that is calcium sulfate dihydrate (although only to about half the proportion in OPC) according to some analyses , but is said to be present as anhydrite, the anhydrous form, in some other studies . While the latter reports may be due to a simplistic chemistry – assuming that since Ca, S and possibly O are detected, H cannot be, the point should be capable of definitive resolution through X-ray diffraction, say.

In the context of OPC, no attempt is made at stoichiometric equations when discussing the chemistry because the natural starting materials are of variable composition, the intimacy of the ground materials cannot be perfect, and all such reactions depend on diffusion and so take time. Many reactions are therefore presented symbolically. It is convenient, as in the field of glasses and porcelain, to refer to all compounds as if they were oxide combinations, e.g. 3CaO.SiO 2 , which makes the accounting easier and is non-presumptive about structure and status. In addition, a shorthand form, called “Cement Chemist Notation” (CCN) is in common use for expressing analytical composition: writing C for CaO, A for Al 2 O 3 , S for SiO 2 , H for H 2 O, etc.; and these labels will be used here for simplicity and convenience. However, the use of the notation for compounds should not be taken to imply the existence of distinct, simple oxide phases as such unless the symbol is used in isolation: they have no constitutive meaning.

As indicated above, manufacture involves the firing at 1400–1500 °C of a finely-ground mixture of limestone (calcium carbonate) and clay silicate minerals such that the main reaction is the formation of tri- and di-calcium silicates, after the decomposition of the limestone:

CaCO 3 ⇒ CaO + CO 2
CaO+MO.SiO2.xH2O3CaO.SiO2,2CaO.SiO2+MO+xH2O⌃︀
CaO + MO . SiO 2 . x H 2 O ⇒ 3 CaO . SiO 2 , 2 CaO . SiO 2 + MO + x H 2 O ⌃︀ C C 3 S C 2 S

This firing process or calcination is sometimes known as clinkering, and the output as clinker. Clay minerals also contain a variety of other metals in minor quantities, such as sodium, potassium and magnesium (also expressed as oxides, “MO”), and these will be therefore present in the fired material as, for example, sulfates. Silicate minerals typically also contain a proportion of alumina, and this reacts on firing to form tri-calcium aluminate:

CaO C + A l 2 O 3 A ⇒ 3 CaO . A l 2 O 3 C 3 A
CaO C + A l 2 O 3 A ⇒ 3 CaO . A l 2 O 3 C 3 A
Only gold members can continue reading. Log In or Register to continue

Nov 28, 2017 | Posted by in Dental Materials | Comments Off on “MTA”—An Hydraulic Silicate Cement: Review update and setting reaction
Premium Wordpress Themes by UFO Themes