mineral trioxide aggregate: present and future...

Mineral trioxide aggregate: presentand future developmentsJOSETTE CAMILLERI

Portland cement is used in the building industry to make concrete. It is hydraulic and thus it develops its

properties well in the presence of moisture. The oral environment is inherently wet, however, all dental

materials need a dry field for optimal performance. A mixture of Portland cement and bismuth oxide was

developed and marketed as mineral trioxide aggregate (MTA). The first formulation was gray in color

followed by a white version and eventually by various similar materials, which are all composed of a

tricalcium silicate-based cement and radiopacifier. The MTA was originally suggested for particular clinical

uses, such as root-end filling and perforation repair, where maintenance of a dry field is difficult. The

suggested uses were extended to apexification and dressing over pulpotomies, pulp capping and also as a

sealer cement. The multipurpose use of MTA necessitated the development of improved formulations

where material additives were included to the original cement/radiopacifier formulation. These additives

supposedly enhance the material’s properties and improve its function. Furthermore, the original Portland

cement formulation was replaced with tricalcium silicate to avoid the inclusion of an aluminum phase and

eliminate the inclusion of trace elements. The bismuth oxide was also replaced with alternative

radiopacifiers. The multifaceted use of MTA allows the material to come into contact with different oral

environmental conditions, including blood, saliva, tissue fluids, dental restorative materials, tooth structure

and even air. These different conditions affect the properties and setting of the material. Future

developments should further investigate the material’s interactions and strive to create materials which are

suited to a specific use.

Received 6 March 2015; accepted 22 March 2015.

Introduction

Mineral trioxide aggregate is a mixture of Portland

cement and bismuth oxide. The physical properties

of most dental materials deteriorate when in contact

with moisture. Portland cement, used in the con-

struction industry, has hydraulic properties and was

therefore introduced as a dental material in order to

counteract this problem. The bismuth oxide was

added to Portland cement to enhance the material’s

radiopacity (1).

Portland cement is manufactured from limestone

and shale. These raw materials are burnt in a kiln to

produce tricalcium silicate, dicalcium silicate, and tri-

calcium aluminate. Other trace oxides, namely mag-

nesium oxide and phosphorous pentoxide, are

commonly found with calcium carbonate mineral

deposits. Other accessory oxides in Portland cement

are sodium oxide, potassium oxide, titanium oxide,

manganese oxide, nickel oxide, barium oxide, chro-

mium oxide and fluoride. These are usually in quan-

tities lower than 0.5%, and many lower than 100

ppm. The sodium oxide, potassium oxide, sulfates

(from the raw materials or fuel) and fluoride act as

fluxing agents, reducing the firing temperature for

Portland cement and therefore reducing the cost of

the cement production. Traces of arsenic, lead and

chromium are also present in the material (2). The

clinker is interground with calcium sulfate thus

forming Portland cement.

The first reported use of Portland cement in

dental literature dates to 1878, when Dr. Witte in

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Endodontic Topics 2015, 32, 31–46All rights reserved

© 2015 John Wiley & Sons A/S.

Published by John Wiley & Sons Ltd

ENDODONTIC TOPICS

1601-1538

Germany published a case report on using Portland

cement to fill root canals (3). Portland cement was

invented in 1824 by Joseph Aspdin in England.

Over a century later Dr. Mahmoud Torabinejad at

Loma Linda University and his co-inventor Dean

White obtained two US patents (4,5) for a Portland

cement-based endodontic material, which became

known as Mineral Trioxide Aggregate (MTA).

The original MTA was dark gray in color and

was commercialized as ProRoot� MTA (Dentsply,

Tulsa Dental, Johnson City, TN, USA) in 1998. In

2002, the “tooth-colored ProRoot MTA” was

introduced and later patented (6). The gray and

white versions of ProRoot MTA have similar com-

positions, but the tooth-colored ProRoot MTA

uses white Portland cement, which contains less

iron. Iron is part of the calcium alumino-ferrite

phase in Portland cement. During manufacture the

ferrite phase functions as a fluxing material, thus

keeping the temperatures during clinkering low,

and results in a more efficient manufacturing pro-

cess. Clinkering of white cement requires the addi-

tion of a fluxing agent. Usually alumino-silicate is

added to the raw feed and allows material clinker-

ing at low temperatures. Regardless of the presence

of alumino-silicate in the white cement, the bio-

compatibility of the gray and white versions has not

been altered (7). However, in contrast other

researchers have reported that white MTA exhibited

less biocompatibility when tested using osteosar-

coma cells than the gray version (8).

Composition of the unhydratedmaterial

The original MTA patents were registered in 1993

and 1995 (4,5) and stated, ‘MTA consists of 50-75%

(wt) calcium oxide and 15-25% silicon dioxide. These

two components together comprise 70-95% of the

cement. When these raw materials are blended, they

produce tricalcium silicate, dicalcium silicate, trical-

cium aluminate, and tetracalcium aluminoferrite’.

They also stated, ‘MTA is Type 1 Portland cement

(American Society for Testing Materials), with a fine-

ness (Blaine number) in the range of 4500-4600 cm2/

g. A radiopacifier (bismuth oxide) is added to the

cement for dental radiological diagnosis’.

Scanning electron microscopy (SEM) of polished

sections of unhydrated MTA embedded in resin shows

distinctive cement grains and bismuth oxide particles,

which are separated from one another. This feature

indicates that the material is composed of a simple mix-

ture of the two components (Fig. 1). The elemental

composition of MTA as shown by energy dispersive

spectroscopy (EDS) indicates the presence of calcium,

silicon and oxygen with minor peaks for aluminum,

potassium, magnesium and bismuth.

The phases present in MTA are determined by

X-ray diffraction analysis. Using this method of

analysis, unhydrated MTA exhibits peaks for trical-

cium silicate, dicalcium silicate and bismuth oxide.

Using a copper Ka tube, each phase has a particular

pattern, as shown in Figure 2, which can be subse-

quently searched and matched with data derived

from the International Centre of Diffraction. MTA

contains other phases such as dicalcium silicate and

tricalcium aluminate in minimal quantities. X-ray

diffraction analysis of MTA eliminates the myth that

MTA is composed mainly of oxides. Thus, the term

“trioxide aggregate” is essentially a misnomer. In

the first publication on MTA composition, calcium

phosphate was stated to be the main constituent of

MTA (9). In the 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% sil-

ica, the remainder being oxygen. In areas of amor-

phous structure, there seemed to be 33% calcium, 49%

Fig. 1. Back scatter scanning electron micrograph ofpolished surface of MTA powder 800X magnification.Reproduced from Camilleri (11).

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Camilleri

phosphate, 2% carbon, 3% chloride, and 6% silica”.

This was discredited in a later publication where it

was shown that the phosphate phase was due to

material contamination from processing for SEM

evaluation (10).

Setting reaction

Mineral trioxide aggregate hydrates when it comes

into contact with water and undergoes two main

reactions. The tricalcium silicate and dicalcium sili-

cate react with water to form calcium silicate hydrate

and calcium hydroxide. The tricalcium aluminate

reacts with water, and in the presence of calcium sul-

fate initially produces ettringite. When the sulfate

containing phases are depleted, a monosulfate phase

is formed (11, 12).

The reactions that occur in MTA after hydration

are the same reactions which are seen in Portland

cement. During the initial stages of the reaction,

calcium silicate hydrate is formed, coating the

cement particles and preventing further reactions.

Tricalcium aluminate dissolves and reacts with the

calcium and sulfate ions present in the liquid phase

to produce ettringite, which 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 hours, which is a

period of relative inactivity and the cement is plastic

and workable. Following the completion of the dor-

mant period, setting of the cement proceeds to the

acceleration stage, where the hydration process accel-

erates 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. Sulfate ions are depleted and

monosulfate forms from ettringite. Crystalline

calcium hydroxide also precipitates from the liquid

phase (11).

Fig. 2. X-ray diffractogram of unhydrated MTA showing the main phases present.

2ð3CaO.SiO2Þ þ 6H2O �! 3CaO.2SiO2:3H2Oþ 3Ca(OH)2tricalcium silicate + water calcium silicate hydrate + calcium hydroxide

2ð2CaO.SiO2Þ þ 4H2O �! 3CaO.2SiO2:3H2Oþ Ca(OH)2dicalcium silicate + water calcium silicate hydrate + calcium hydroxide

3CaO.Al2O3 þ CaSO4 þH2O �! 3CaO.Al2O3:3CaSO4:31H2Otricalcium aluminate + gypsum + water ettringite

33

MTA: present and future developments

Reaction by-products produced upon hydration of

the cement are deposited around the periphery of

the unhydrated cement particles. As hydration pro-

ceeds, there is evidence of more reaction by-products

(11,13). 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 elec-

tron micrographs of polished cement specimens

(Fig. 3a). The formation of cement by-products can

be monitored by scanning electron microscopy. Cal-

cium silicate hydrate exhibits a typical honeycomb

appearance, while calcium hydroxide is deposited in

the form of hexagonal plates (Fig. 3b).

Material hydration can be monitored using X-ray

diffraction analysis, since most of the hydration by-

products are crystalline. As hydration progresses, a

rise in calcium hydroxide peak (ICDD: 44-1481) is

seen at 18 and 34°2h using a copper Ka X-ray radia-

tion. The unhydrated tricalcium silicate phase

(ICDD: 86-0402) at 29.4, 32.1, 32.5, 32.6, 34.3

and 41.2°2h usually reduces in intensity (Fig. 4).

Quantitative measurements of the different phases of

MTA may be performed using Rietveld refinement

of X-ray diffraction plots (Table 1). This quantitative

assessment verifies the reduction in tricalcium silicate

and dicalcium silicate levels of the unhydrated

cement as hydration proceeds with the formation of

calcium silicate hydrate. Calcium hydroxide and low

levels of ettringite are also produced (13).

If MTA is left exposed to the environment, the

calcium hydroxide reacts with the atmospheric car-

bon dioxide resulting in the deposition of calcium

carbonate on the cement surface (Fig. 5a). These

deposits are commonly mistaken as being an integral

part of the cement microstructure (7). When in con-

tact with tissue fluids and synthetic tissue fluids,

which contain phosphate ions, the calcium hydroxide

produced, as a by-product of cement hydration,

reacts to form calcium phosphate and is deposited

on the cement surface. Calcium phosphate is crystal-

A

Fig. 3a. Back scatter scanning electron micrograph ofhydrated MTA after immersion for 28 days in watershowing unhydrated cement particle enclosed by a reac-tion rim. Shiny particles are bismuth oxide (X 500mag.).

B

Fig. 3b. Secondary electron scanning electron micro-graph of MTA surface showing calcium silicate hydrate(honeycomb appearance) and calcium hydroxide (hex-agonal plates) deposited on its surface (X 2500 mag.).

Table 1: Quantitative analysis using Rietveld X-raydiffraction of the phases present in unhydrated andhydrated ProRoot MTA. Reproduced from Camilleri(13).

Phases identified

Material type in mass %

Unhydrated Hydrated

Tricalcium silicate 53.1 10.6

Dicalcium silicate 22.5 14.9

Anhydrite 1.5 0.0

Calcium hydroxide 0.0 14.4

Calcium silicate hydrate 0.0 49.5

Ettringite 0.0 2.1

Bismuth oxide 21.6 8.4

34

Camilleri

line and its presence can be demonstrated by X-ray

diffraction analysis (Fig. 5b). The formation of cal-

cium phosphate on the material surface has been

reported to be the reason for the bioactivity shown

by MTA (Fig. 6). This phenomenon is well

researched (14–18).

Limitations of the originalformulation

The original formulation of MTA is composed of

Portland cement and bismuth oxide in a 4:1 propor-

tion, as specified by the MTA patent (4–6), and mar-

keted by Dentsply as ProRoot MTA, originally as

gray MTA and later as white. Some concerns have

been raised with the original formulation for a num-

ber of reasons and these are listed below.

Trace element inclusion

Since the main constituent of MTA is Portland

cement, some concerns have been raised about the

inclusion of trace elements. Portland cement is man-

ufactured from naturally occurring raw materials in a

kiln. Sometimes, waste materials are used as the raw

materials in the manufacturing of Portland cement

for the construction industry. Additionally, waste

materials are also used as a replacement for primary

fuels in order to keep the manufacturing costs low.

This potentially leads to the inclusion of trace ele-

ments in the final product. Traces of arsenic, lead

and chromium have been reported in both gray and

white MTA (19–25). Although the levels of acid-

extractable trace elements are high, the amounts lea-

ched in the solution are negligible. The inclusion of

heavy metals in Portland cement has raised concerns

regarding its use as a base material for MTA because

when it is applied, the MTA is in contact with both

hard and soft human tissues.

Presence of aluminum

Aluminum is part of the calcium aluminate phase of

the Portland cement. This phase is an integral part

of the cement and cannot be removed as it is pro-

duced during the cement manufacture. Aluminum

is not indorsed in dental and biomaterials due to

risk of Alzheimer’s from excessive exposure to alu-

minum in close contact with the human tissues

(26).

Stability and leaching of bismuth

Bismuth oxide is added to MTA to act as a radiopa-

cifying agent since Portland cement is not sufficiently

radiopaque. Quantitative X-ray diffraction analysis

Fig. 4. X-ray diffractogram of unhydrated and hydrated MTA after 1 and 28 days showing the main phases pres-ent. BO: bismuth oxide, P: Portlandite/calcium hydroxide, TCS: tricalcium silicate.

35


(Table 1) identifies a reduction of bismuth oxide

after cement hydration, with 8.4% present in the

hydrated cement as opposed to the 21.6% in the

unhydrated material (13). Atomic ratio plots of Bi/

Ca versus Si/Ca demonstrate a linear relationship

between bismuth and silicon; the result infers that

bismuth is replacing silicon in the calcium silicate

hydrate structure (13).

Bismuth oxide has also been implicated in tooth

discoloration. Tooth discoloration has been reported

with the use of both gray and white MTA. Most of

the reports were incidental findings while investigat-

ing the efficacy for multifactorial use of MTA (27–

32). Gray MTA led to clinically perceptible crown

discoloration after 1 month, while the total color

change caused by white MTA exceeded the percepti-

ble threshold for the human eye after 3 months.

This suggests that the application of gray MTA in

the aesthetic zone should be avoided, while white

MTA should be used with caution when filling pulp

chambers with the materials (33). The causative fac-

tors implicated in the change in color of bismuth

oxide from yellow to dark brown (Fig. 7) include

reaction with sodium hypochlorite (34), formalde-

hyde (35) and also contact with collagen thus tooth

structure (36). Light and the presence of oxygen

have also been shown to be precipitating factors

leading to material discoloration (37,38).

Material manipulation and other physicalproperties

Another main concern with the original MTA for-

mulation is the consistency of the freshly mixed

material, which is usually termed as grainy or sandy.

The original formulation is also hand spatulated and

A

Fig. 5a. Scanning electron micrographs showing carbonation of MTA surface.

36

Camilleri

delivery to the operative site is difficult. MTA pre-

sents longer setting time (2 hours and 45 minutes)

in comparison to other retrograde filling materials

such as amalgam, Super-EBA and IRM (9). The

long setting time makes MTA unsuitable for most of

the clinical applications. Different powder-to-water

ratios, which tend to be a frequent occurrence in

clinical practice since the gram supplied is usually

not used in one application, also exerts influence on

MTA’s properties and promoting a longer setting

time (39,40).

Developments

More MTA formulations were introduced after the

ProRoot by Dentsply. The first was introduced by

Angelus, which had both gray and white formula-

tions. The MTA Angelus is also composed of Port-

land cement and bismuth oxide. However the

quantities of tricalcium and dicalcium silicate in

MTA Angelus are different to ProRoot and other

Portland cements (13,41). The difference is due to

variations in the manufacturing of Portland cement,

used as a raw material for preparing MTA. Quantita-

tive phase analysis also showed a percentage of cal-

cium oxide which is responsible for the initial high

heat flux exhibited on calorimetry (41). This calcium

oxide is a result of a raw mix with poor combustibil-

ity and with an unstable thermal profile. Further-

more MTA Angelus was found to contain tricalcium

aluminate but no sulfate-containing phase (Table 2).

The manufacturer claims that the absence of gypsum

reduces the setting time of the material. In fact,

MTA Angelus has been shown to set in less than 50

minutes (42), as opposed to ProRoot MTA which

was reported to have a setting time of over 2 hours

(9,43,44). MTA Angelus also exhibits a lower level

of bismuth oxide when compared to ProRoot MTA

(Table 2) which accounts for the lower radiopacity

of MTA Angelus (18).

More recently, MTA Plus was introduced by an

American company, Avalon Biomed. Both ProRoot

MTA and MTA Angelus were found to have a simi-

lar fineness (1 m2/g) when tested using the Bru-

nauer–Emmett–Teller (BET) gas adsorption method

(12,41). MTA Plus has a specific surface area of

1.537 m2/g, which is higher than the values

obtained for the other MTAs (12). The higher spe-

cific surface area allows more surface availability for

the cement reaction and consequently results in a

more rapid reaction rate.

Replacement of Portland cement

The associated risks to lead, arsenic and chromium

exposure, coupled by the presence of aluminum led

B

Fig. 5b. Glancing angle X-ray diffractogram of MTA analyzing surface deposits on MTA after immersion in waterand Hank’s balanced salt solution for 28 days. In Hank’s balanced salt solution deposits of calcium phosphate wereshown on the cement surface.

37


A

C D

B

E F

G

Fig. 6. Characterization of precipitates formed by MTA BIO after 2 months of immersion in phosphate-bufferedsaline. A. SEM image showing the acicular nature of spherules (original magnification, 8000x). B. Energy dispersiveX-ray (EDAX) spectrum for precipitates in (A) and semi-quantitative chemical composition showing their Ca/P

38

Camilleri

to the development of materials based on pure trical-

cium silicate. Tricalcium silicate cement is manufac-

tured with laboratory-grade materials using the

sol-gel method. The lack of natural minerals, and

wastes used to fire the kiln, reduces the risk of trace

element contamination.

Materials that use tricalcium silicate as their main

cementious phase include Biodentine and BioAggre-

gate. Tricalcium silicate cement hydrates in a similar

way to Portland cement and produces calcium

hydroxide on hydration (45). The tricalcium silicate-

based materials are collectively known as bioceram-

ics, which distinguishes them from the Portland

cement-based materials and thus signifying a purer

product. In fact, this is incorrect, since the term

ceramics encompasses a wider variety of materials

with a different chemistry.

Replacement of bismuth oxide

The newer MTA variants do not use the bismuth

oxide as radiopacifier. Several alternative radiopacifi-

ers have been investigated and most do not impart

the necessary radiopacity to the final material for-

mulation. Addition of gold powder, silver/tin alloy

(46), barium sulfate (46–48), iodoform (47), zir-

conium oxide (39,47), zinc oxide (46,48), lead

oxide, bismuth subnitrate, bismuth carbonate and

calcium tungstate (48) results in a radiopacity value

of more than 3 mm of the aluminum scale. The

commercial materials use zirconium oxide and tan-

talum oxide. The latter is found in BioAggregate.

Zirconium oxide is found in Biodentine, BioRoot

RCS and EndoSequence BC Sealer and has been

shown to be inert (49). It does not leach and zir-

conium does not take part in the material hydra-

tion to form zirconium-based complexes. Other

alternative radiopacifiers such as barium sulfate and

metallic components, like silver–tin alloys, leach in

to the solution and are therefore not ideal replace-

ments (50). Although gold is inert and imparts

adequate radiopacity, its cost and color limits its

use. Leaching of barium has been demonstrated

from barium zirconate and even when used in

conjunction with tricalcium silicate cement with a

resin matrix (51).

molar ratio. C. SEM image showing petal-like precipitates (original magnification, 1000x). D. EDAX spectrum forprecipitates in (C) revealed a greater Ca/P molar ratio and lattice substitution of Na and Cl. E. SEM image of com-pact lath-like precipitates (original magnification, 1000x). F. Semi-quantitative analysis of the EDAX data derivedfrom (E) indicates that the precipitates have a Ca/P molar ratio of 1.61 with lattice substitution of Na, Cl, andMg. G. X-ray diffraction pattern of the calcium phosphate precipitates obtained after 2 months of immersion inphosphate-buffered saline, revealing the presence of poorly-crystalline apatite. Reproduced from Reyes-Carmona etal. (17).

Fig. 7. Photographs of bismuth oxide powder, ProRoot MTA and Portland cement before and after immersion inthe different solutions. Reproduced from Camilleri (34).

39


Improvement in the physical properties,material manipulation and suitability forspecific use

One of the main clinical concerns with the original

MTA formulation is the mixing and delivery system.

Classically, MTA is mixed by manipulating the pow-

der and liquid components on a mixing pad. Alter-

native mixing techniques, such as the use of an

amalgamator, have been investigated (52–54). MM-

MTA, manufactured by Micro-Mega (Besanc�on Ce-

dex, France), uses MTA capsules that enable the

MTA to be mixed using an amalgamator. Biodentine

also uses capsules. The advantage of the MM-MTA

delivery system is that the capsule incorporates a

delivery system similar to what is available with glass

ionomer cements. The effectiveness of mechanical

mixing is not clear, although the compressive

strength of the set material was shown to be

enhanced (52). In addition to enhanced material

microhardness (53), other research has shown that

the various mixing methods have no significant

effects on the resultant MTA mixtures (54).

Another clinical concern with the use of the origi-

nal MTA formulation is the setting time, which was

reported to be more than 3 hours (9). A long set-

ting time is not an issue if the material is being used

for root-end filling and perforation repair. However

when used as a pulp capping material and as dressing

over pulpotomies a shorter setting time is necessary.

There have been a large number of reports regarding

the addition of various chemicals to the mixing

liquid or the substitution of the water for other liq-

uids. The most popular chemicals are calcium chlo-

ride (44,55–57), calcium nitrite/nitrate and calcium

formate (57). These additions are also used in the

construction industry to accelerate the setting of

Portland cement. Setting accelerators affect the set-

ting reaction of both tricalcium silicate and trical-

cium aluminate (58). Biodentine and MM-MTA

have a reduced setting time and both use calcium

chloride accelerator in their systems (59,60).

The development of resin modified tricalcium sili-

cate-based materials result in a command cure mate-

rial which is useful for pulp capping. Resins also

result in increased material flow consequently mak-

ing the material suitable as a root canal sealer. The

mixing liquid, which is usually water, has been

replaced by various resins. These modifications have

resulted in the development of light-activated MTA

and resin-modified MTA for use in root canal sealer

cements. A number of resin systems have been inves-

tigated. The main ones being light-curing systems,

containing Bisphenol A-glycidyl methacrylate

(Bis-GMA), and a biocompatible resin (61,62)

consisting of 2-hydroxyethyl methacrylate (HEMA),

Table 2: Quantitative analysis using Rietveld X-ray diffraction of the phases present in unhydrated Portlandcement, ProRoot MTA and MTA Angelus

Phases identified

Material type in mass %

Portland cement ProRoot MTA* (∞) MTA Angelus~

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 oxide 0.0 0.0 8.0

Bismuth oxide 0.0 21.6 (19.8) 14.0

* Camilleri (13).∞ Bel�ıo-Reyes IA, Bucio L, Cruz-Chavez E. Phase composition of ProRoot mineral trioxide aggregate by X-ray powderdiffraction. J Endod 2009: 35: 875-878.~ Camilleri et al. (41).

40

Camilleri

triethylene glycol dimethacrylate (TEGDMA), cam-

phorquinone and ethyl-4-(dimethylamino)benzoate

(EDMAB), with or without polyacrylic co-maeleic

acid (63,64), Bis-GMA, TEGDMA (65) and

Bis-GMA, pyromellitic acid diethylmethacrylate

(PMDM) and HEMA (66). Chemically-cured resins

have also been employed (65,67) and investigated

in vitro. The addition of different resin systems has

shown to alter the hydration mechanism of the

resultant material. Resins are mostly hydrophobic

and limit ion exchange with the surrounding envi-

ronment. This restricts the absorption of fluids and

reduces cement hydration, therefore the calcium

ions, even if formed, would find it difficult to move

along the resin matrix. This modification of hydra-

tion and restricted ion movement has been reported

(51). Thus if hydration is incomplete the use of resin

modified tricalcium silicate is useless as no calcium

hydroxide is released to interact with the pulp.

One light curable tricalcium silicate-based material

marketed for pulp capping is Theracal (Bisco,

Schaumburg, IL, USA). Theracal hydration is

affected by the environmental conditions and fluid

availability. In fact when used as a pulp capping mate-

rial the low fluid availability reduces material hydra-

tion (68). The calcium releasing ability of Theracal is

debatable, in one study it was shown to be compara-

ble to calcium hydroxide (69), while in later investi-

gations no calcium was formed and calcium ion

leaching was shown to be very low compared to

Biodentine (70). Theracal extracts were shown to be

cytotoxic when in contact with pulp cells (71). A

modified MTA-based resin is marketed by Angelus

(Angelus, Londrina, Brazil) as MTA Fillapex. This is

composed of MTA, a salicylate resin (methyl salicy-

late, butylene glycol and colophony) and other addi-

tives. Other proprietary brands such as MTA Obtura

and Endo CPM sealer exist and contain other addi-

tives to the MTA formula which enhances material

flow. In MTA Fillapex, the hydration reaction does

not occur and therefore no calcium hydroxide is lea-

ched to the surrounding tissues (72).

Material flow can be enhanced by the addition of

water-soluble polymers (73–76) as shown in proto-

type materials. ProRoot Endo Sealer is a commercial

formulation using cement particles dispersed in a

water-soluble polymer (77,78). The water-soluble

polymer creates a charge on the cement particles,

resulting in the repulsion of these charged particles,

with reduced flocculation and increased material flow

at low water/cement ratios (58). Biodentine and

BioRoot RCS are both manufactured by Septodont

and have a water soluble polymer which improves

the material handling. Propylene glycol, another sol-

uble polymer, has also been used to improve MTA

flow (79).

Other polymers have been included to reduce the

washout of the unset cement. Anti-washout liquid is

included in the MTA Plus formulation (Avalon Bio-

med Inc., Bradenton, FL, USA). Washout refers to

the tendency of a freshly prepared cement paste to

“disintegrate upon early contact with blood or other

fluids” (80) and is one of the disadvantages of MTA

after it is placed in situ. The addition of an anti-

washout liquid to MTA Plus reduces washout con-

siderably when compared to MTA Angelus (81).

Intermediate Restorative Material (IRM) and dental

amalgam exhibit no washout (82).

Most of the newer formulations contain additives

which enhance material properties. Both Biodentine

and MM-MTA include calcium carbonate and is a

component of Portland cement when used in the

building industry. The addition of calcium carbonate

reduces the material costs and acts as a nucleating

agent, thus enhancing hydration. This has been

demonstrated for Biodentine, where the hydration

product was shown to deposit around the calcium

carbonate particles (83,84). In MM-MTA the cal-

cium carbonate changes the hydration reaction.

Since MM-MTA is Portland cement-based it con-

tains an aluminate phase. The calcium carbonate

affects the formation of ettringite, which is a by-

product of the hydration of tricalcium aluminate,

calcium sulfate and water during hydration, conse-

quently modifying the hydration process. During the

formation of ettringite, the sulfate ions are replaced

by carbonate ions producing calcium carbosilicate

and calcium carboaluminate (60). Calcium carbonate

has been also shown to impede crystallization of cal-

cium hydroxide. In fact no formation of calcium

hydroxide was demonstrated for MM-MTA (60).

BioAggregate, EndoSequence BC Sealer and the

material range from Brasseler contains calcium phos-

phate monobasic (84,85). There is no literature on

the effect of adding phosphate containing materials

to tricalcium silicate-based materials, however a

reduction in the formation of calcium hydroxide

over a period of 28 days was demonstrated for Bio-

41


aggregate (84). MTA Fillapex contains microsilica,

which affects the hydration of Portland cement. It

reacts with the calcium hydroxide produced during

hydration and forms more calcium silicate hydrate in

the long term and improves the material properties

at the expense of calcium hydroxide formation.

However, MTA Fillapex does not produce calcium

hydroxide as shown by X-ray diffraction analysis

(72). The different additives may potentially affect

the cyto-compatibility of the materials since different

calcium silicate-based materials exhibited different

cytotoxicity to human gingival fibroblasts (85).

Material use and effect of environmentalconditions

Although MTA is a hydraulic material, when used in

clinical dentistry it never comes in contact with

water. The material has been suggested for use as a

root-end filling material, for perforation repair, apex-

ification and pulpotomy procedures and also as a

pulp capping material (87). More recently MTA has

also been developed as a root canal sealer. All the

beneficial properties of MTA are related to its

hydraulic nature and the formation of calcium

hydroxide, as a by-product of material hydration.

Each material application brings MTA in contact

with different environments. Blood comes into con-

tact with MTA when it is used for perforation repair,

as a root-end filling material and to a minor extent

in cases of direct pulp capping. Blood interferes with

material hydration and reduces the MTA microhard-

ness (88–90). Tissue fluids and serum also affect

material properties (91) and failure of material set-

ting (92). The phosphates present in tissue fluids

have been implicated in material bioactivity, as reac-

tion of calcium hydroxide with phosphates results in

the formation of hydroxyl-apatite on the material

surface (14–16). However phosphates have also been

shown to interfere with material setting (12).

The interaction with pulp and dentin also provides

an interesting and beneficial phenomena. MTA

induces pulp healing and regeneration (93). How-

ever MTA does have several drawbacks. The flexural

strength of the tooth when in contact with MTA

was reduced (94) and MTA’s interaction with colla-

gen also caused tooth discoloration. (36). Further-

more the long setting time of MTA makes unset

MTA come in contact with other dental materials

affecting the material hydration and setting (95,96).

The development of tricalcium silicate root canal

sealers is based on the hypothesis of the interaction

of dentin with the tricalcium silicate and biominerali-

zation, consequently forming dentin tags. This is

postulated for the hydrated material. MTA Fillapex

does not produce calcium hydroxide after it sets. En-

doSequence, and materials which are hybrids of tri-

calcium silicate and calcium phosphate, may not

produce calcium hydroxide in the long term. Thus

the hypothesis of biomineralization may be errone-

ous. MTA and related materials are affected by vari-

ous irrigants used during root canal treatment (97),

particularly EDTA which is a calcium chelator (98).

Material and tooth discoloration will result from the

interaction of sodium hypochlorite with bismuth

oxide in MTA (34).

Conclusions

For the past two decades MTA and tricalcium silicate

cement-based materials have become a mainstay in

clinical dentistry and are used for a variety of clinical

applications. The original formulation and modified

cements are still used in these applications. The

materials are reactive and additives tend to affect the

hydration process, which can be deleterious to the

material chemistry and resultant properties of the set

material. Furthermore the materials interact with the

oral environment during use, which changes material

microstructure and even demonstrates setting failure.

Future developments in MTA and related materials

should outline first the particular material use and

then tailor-make the material according to the

desired properties.

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