An Updated Review Of Mineral Trioxide Aggregate Part-1 :Compositional Analysis, Setting Reaction And Physical Properties

by

Shahbaz Khan1                                                                 BDS, MPhil (Scholar)

Muhammad Amber Fareed2                                       BDS, MSc, PhD

Muhammad Kaleem3                                                   BDS, MSc, PhD

Shahab Ud Din3                                                                 BDS, MSc, PhD

Kefi Iqbal4                                                                         BDS, MSc, PhD

ABSTRACT: The aims of Part-1 updated review are present the chemical composition, setting reaction, mechanism of action and physical properties of Mineral Trioxide Aggregate (MTA). MTA is a biocompatible and bioactive material which gained rapid acceptance in the field of dentistry. The powder of MTA contains fine hydrophilic particles (1.0-30 µm) of calcium silicate phases and bismuth oxide whereas; different liquids have been used to hydrate MTA powder. Several methods have been reported for compositional analysis including energy dispersive analysis with X-ray (EDAX), inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray diffraction analyses (XRD), X-ray fluorescence spectrometry (XRF), energy x-ray spectrometry and energy dispersive spectroscopy. When MTA powder is mixed with water, calcium hydroxide (CH) and calcim silicate hydrate (C-S-H) are initially formed and eventually transform into a poorly crystallized and porous solid gel. The mineral phases of MTA include dicalcium silicate (C2S), tricalcium silicate (C3S), tricalcium aluminate (C3A) and tetracalcium aluminoferrate (C4AF) which reacts with water to produce calcium silicate hydrate (C-S-H) and calcium hydroxide. The physical properties of MTA are influenced by the storage media, powder/(C-S-H) ratio, method of mixing, condensation pressure, humidity, the type of MTA, environmental pH, the length of time between mixing and evaluation, thickness of the material and temperature during setting. Generally, MTA has a long setting time, high pH, low compressive strength and possesses antibacterial and antifungal properties.

KEY WORDS: Mineral trioxide aggregate, composition, setting reaction, mechanism of action, physical properties.

HOW TO CITE: Khan S, Fareed MA, Kaleem M, Uddin S, Iqbal K. An Updated Review of Mineral Trioxide Aggregate Part-1: Compositional Analysis, Setting Reaction And Physical Properties. J Pak Dent Assoc 2014; 23(4):140-147

INTRODUCTION

Majority of endodontic failures results due to leakage of irritants from pathologically involved root canals. When a conventional non-surgical procedure fails to save the tooth, surgical endodontic therapy is indicated. The outcome of surgical endodontic procedures relies on the complete prevention of bacterial leakage from root canal system into periapical tissues. Therefore, selection of a suitable endodontic filling material is of great importance for successful
endodontic treatment1. Over the years several materials were developed and suggested for surgical endodontic applications such as, amalgam, gutta percha, zinc phosphate cement, polycarboxylate cement, zinc oxide eugenol paste, ethoxy benzoic acid (EBA) cement, glass ionomer cements, composite resins and mineral trioxide aggregate (MTA)1,2. MTA was pioneered by Torabinejad and White for root end filling and endodontic repair procedures3. The
novel material was patented in 19954 and approved for endodontic applications in 19985 having commercial name ProRoot MTA (Tulsa Dental Products, Tulsa, OK, USA)6 . MTA was initially introduced in gray form (GMTA), however due to discoloration potential of GMTA white form of MTA (WMTA) was developed7.

Since inception, MTA rapidly gained acceptance among dentists and has been extensively investigated as a potential material to seal the pathways of communication between the tooth root canal system and the external surfaces8. Due to superior biocompatibility, bioactivity and sealing ability9, MTA is used for both surgical and non-surgical endodontic applications8.

The aim of part-1 updated review is to emphasize the current knowledge of compositional analysis, material characteristics, setting behavior, mechanism of action and physical properties of MTA. Whereas, part-2 updated review draw attention to the clinical applications of this promising material in root-end filling, perforation repair, vital pulp therapy, and apical barrier formation in addition to the comprehensive comparison of other MTA alternative materials available commercially. Therefore, a systematic research of previously published work in PubMed/MEDLINE (National Library of Medicine, Bethesda, MD), Scopus and Google Scholar databases were conducted from 1995 to November 2014 using different combinations of the following key words: “mineral trioxide aggregate”, “composition”, “clinical applications”, “mechanism of action”, “physical and biological properties”. The literature was screened by authors for relevancy and key findings of the current concepts of MTA are reported here.

2. MATERIAL CHARACTERISTICS

2.1 Composition of MTA

MTA was inspired and derived from an ordinary Portland cement (PC)10 therefore; MTA’s chemical composition is considerably similar to Portland cements. MTA is a complex chemical compound composed of various mineral phases which comprised of simple oxides of various elements11. Therefore composition of unset MTA powder is generally evaluated in terms of elemental composition, presence of simple oxides and mineral phase composition. Several studies evaluated elemental composition of MTA and reported that calcium, silica, bismuth and oxygen comprise the main elements present in MTA6,12,13. According to the MTA patent, calcium and silica are the main reactive elements whereas bismuth was added for radiopacity8,10. Both gray and white variant of MTA contains similar elements, except for the presence of iron in GMTA and comparatively low amounts of aluminum in WMTA6,13-16 (Tab. 1).
According to Torabinejad and White, the experimental

Tab. 1: Elemental composition, simple oxides and mineral phases of
GMTA and WMTA.

material used in the patent consisted of calcium oxide (50-75 wt %), silicon oxide (15-25 wt %) and aluminum oxide, together these oxides constituted 70-95 wt % of MTA10. The compositional imaging of both types of MTA presented oxygen distribution throughout crystalline and amorphous phases which indicated that all elements in MTA were present in their oxide form13. Furthermore, Asgary et al., evaluated the chemical differences between both variants of MTA and reported that WMTA contained relatively less amounts of iron oxide, aluminum oxide and magnesium oxide. The authors interpreted that less amount of iron oxide is responsible for the white color of WMTA13. The oxides of chromium, iron and copper which have free d electrons have strong colors. Whereas, oxides of elements where electrons cannot be easily excited such as aluminum, silicon, calcium and titanium are either colorless or white12. When the aforementioned oxides are blended together they produce mineral phases such as dicalcium silicate (C2S), tricalcium silicate (C3S), tricalcium aluminate (C3A) and tetracalcium aluminoferrate (C4AF)17. Song et al. reported that crystal structure of both types of MTA were similar and chemically contained calcium silicate phases and bismuth oxide14. Similarly, Camilleri et al., reported that X-ray diffraction analysis (XRD) showed both variants of MTA were purely crystalline and composed of identical mineral phases. The WMTA was primarily composed of C3S and bismuth oxide, while in GMTA, C3S, C2S and bismuth oxide constituted primary mineral phases6.

2.2 Particle morphology of MTA

Particle size and morphology of a biomaterial is important because it significantly affects the physical properties. For hydraulic materials such as MTA, a smaller particle size results in more surface area available to react with water which accelerates the setting reaction and provides greater early strength7. Moreover, the uniform particle size distribution have higher mechanical properties due to reduction of spreading in grit size18.
Many studies have investigated particle size and shape of MTA and reported that WMTA particles were finer in comparison to GMTA6,13,19. MTA particle size range is less than 1 µm to approximately 30 µm however; occasionally particles up to 50 µm were also reported. The particle size of bismuth range between 10-30 µm20.
Asgary et al., reported particles of 5-50 µm in GMTA and 5-25 µm in WMTA13. Camilleri et al. reported that both types of MTA contained irregular particles. WMTA contained small irregular particles with some elongated

Fig. 1: SEM images showing particle size and morphology of (a) WMTA and (b) GMTA at 350x respectively6

needle like particles whereas, GMTA contained large irregular particles along with small as well as elongated particles (Fig. 1)6.

3. SETTING REACTION OF MTA

The setting reaction of MTA is a complicated process depending on the exact proportions of mineral phases, their purity and temperature of the mix21. On hydration calcium silicates present in MTA undergoes hydrolysis and produce calcium silicate hydrate and calcium hydroxide21. About one third of hydration products constituted by calcium hydroxide22 to renders MTA highly alkaline20. On hydration any excess calcium oxide readily reacts to form calcium hydroxide as:
CaO + H2O g Ca(OH)2 …21
Whereas, C2S and C3S react with water to produce calcium silicate hydrate (C-S-H) and calcium hydroxide
as:
2(3CaO.SiO2) + 6H2O g 3CaO2.SiO2.3H2O + 3Ca(OH)2 …22
and,
2(2CaO.SiO2) + 4H2O g 3CaO2.SiO2.3H2O + Ca(OH)2 …22
The C3S is most important mineral phase in MTA and engages in the formation of C-S-H to provide early strength. On the other hand, C2S reacts relatively slow and give later strength to the set material23. C3A present in MTA reacts with water to form calcium aluminates and (in presence of calcium sulphate) sulfate aluminates21.
The C-S-H, the major hydration product of MTA is an amorphous compound with varying stoichiometric values. The Ca:Si ratio in C-S-H generally varies between 0.8 and 2.1 with highly variable content of water23 therefore, set MTA can be described as calcium hydroxide contained within a silicate matrix6.

4. MECHANISM OF ACTION

The successful usage of MTA in endodontic applications can be attributed to its biocompatibility, bioactivity and mechanism of action. Parirokh and Torabinejad24 summarized the four actions of MTA after direct placement in contact with living tissues (Fig. 2);
(i) Creation of an inhospitable environment for growth of bacteria due to alkaline pH.

Fig. 2: Mechanism of action of MTA.

(ii) Formation of hydroxyapatite like mineral structure on its surface and provide the biological seal.
(iii) Formation of calcium hydroxide which dissociates to release Ca ions to promotes cellular attachment and proliferation.
(iv) Modulation of cytokine production and encouragement of hard tissue forming cells to differentiate and migrate.

4.1 Inhibition of bacterial growth

MTA is a potent growth inhibitor of staphylococcus aureus, enterococcus faecalis and pseudomonas aeruginosa as compared to amalgam, Geristore (Resin modified GIC), Super-Bond C&B (resin cement), Dyract (compomer) and Clearfil AP-X (composite)25. As discussed above, in the setting reaction of MTA, C-S-H and calcium hydroxide forms. The dissociation of calcium hydroxide in calcium and hydroxide ions results in an increased pH of MTA. Therefore, its antimicrobial properties can be attributed to elevated pH26. Torabinejad et al., reported alkaline pH (10.2) of MTA at initial stages after mixing which increases to 12.5 after an elapse of 3 hours27. A pH level of 12.0 can inhibit growth of most microorganisms including resistant enterococcus faecalis28.

4.2 Precipitation of apatite crystals

MTA releases majority of cationic components in tissue fluids and out of all ions, calcium is the most dominant one. Calcium being sparingly soluble in tissue fluids reacts with tissue phosphates and precipitates HA29. The chemical reaction responsible for the formation and precipitation of HA is:

10 Ca+2 + 6(PO4)-3 + 2(OH)-1 g Ca10(PO4)6 (OH)2 . . .29

This chemical reaction is well known in biological calcification processes which is favored at pH of 7.030 and takes place in the presence of biological environment both in-vivo and in-vitro with calcium containing materials31,32. A material which possess an apatitic surface layer in close contact with mineralized tissues can bond chemically to the later31. Therefore, surface precipitation of hydroxyapatite (HA) gradually continues to the internal structure and this may change the overall compositional constitution of MTA29. According to Sarkar et al., a series of physico-chemical reactions are responsible for the sealing ability of MTA. After placement in root canal, MTA dissociates gradually which leads to nucleation of HA crystals and subsequent precipitation of HA fills the microscopic spaces present between MTA and canal walls. A diffusion-dependant reaction between HA surface layer and dentine mineral structure occurs with course of time and the initial mechanical seal between MTA and dentine wall is converted to a chemical one29.

4.3 Cellular response and mineralization

Cellular response to MTA or its extracts have been extensively studied and reported to modulate expression of cytokines and other biological markers33-35. Studies have shown up-regulation of interleukin (IL- 1a , IL1b, IL-2, IL-4, IL-6, IL-8, IL-10, IL-18), osteocalcin, osteopontin, alkaline phosphatase and bone morphogenic protein-233 38. Abdullah et al., studied the effect of MTA, GIC and two variants of PC on expression of IL-1b, IL6, IL-18 and osteocalcin in a direct contact assay model and reported the up-regulation in cultures with direct contact in MTA and both variants of PC.

5 PHYSICAL PROPERTIES OF MTA

5.1 Compressive strength

Reactive phases of MTA possess different hydration rates, C2S hydrates slowly than C3S, therefore mechanical properties of MTA may take several days to reach their maximum7. MTA shows relatively less compressive strength compared to amalgam, glass ionomer cements and composites4,7 which can be influenced by powder/water ratio used, type of the liquid used for mixing, pH of the mixing liquid and the environment and storage conditions7,39-41.

5.2 Push out strength

A material used for repair of perforations should possess sufficient push out strength to resist dislodgement forces which are generated by functioning of the tooth7. MTA posses lower push out strength compared to IRM (reinforced zinc oxide-eugenol cement) and Super EBA (alumina-fortified cement) after immersion in intra canal bleaching materials42. Hydraulic nature and slow hydration rate of C2S makes push out strength of MTA liable to be influenced by pH, humidity, time after mixing and storage conditions7,42-43.

5.3 Porosity

A widely used technique for characterizing the distribution of pore sizes in cement-based materials (mercury intrusion porosimetry) employed for WMTA and white PC showed consistent presence of pores in both materials. However, pore volume observed in WMTA was significantly less compared to white PC44. Porosity in set structure of MTA is affected by powder/water ratio, addition of bismuth oxide, entrapment of air during mixing material and pH of the environment7,45-47.

5.4 Radiopacity

The addition of bismuth oxide renders MTA radiopaque6 which is sufficient to make MTA recognizable on radiographs4. However, radiopacity of MTA is less than amalgam and gutta percha48,49.

5.5 Solubility

Solubility of MTA is influenced by the powder/water ratio used for its mixing. A higher amount of water leads to more porosity and increases solubility in set structure of MTA by causing calcium released in greater amounts46.
Solubility in tissue fluids can jeopardize clinical performance of endodontic materials50. Several researchers have reported that MTA have little or no solubility4,51, however, in contrast, Fridland and Rosado reported increased solubility of MTA in a long term study52.

5.6 Marginal adaptation and sealing ability

Marginal adaptation and sealing ability of endodontic filling materials is of paramount importance since irritants that leaks from infected root canals to surrounding periradicular tissues accounts for majority of endodontic failures50. Several studies reported that MTA holds better marginal adaptation compared to amalgam, Super EBA, IRM and glass ionomer cement (GIC)53-55. The sealing ability of MTA was extensively evaluated by leakage studies (dye leakage, fluid filtration, protein leakage and bacterial leakage) and had shown superior sealing ability for MTA compared to amalgam, IRM and super EBA33,55-57. The setting reaction of MTA is accompanied with setting expansion which may reduce gaps between MTA and dentinal walls58. According to Torabinejad et al., dentine-biomaterial interface of amalgam, super EBA and IRM showed gaps (3.8-14.9 µm) while, the interface was free of any gaps in case of MTA55. A meta-analysis of studies on endodontic filling materials reported better prevention of dye and bacterial penetration for MTA compared to amalgam59, however extend setting reaction of MTA, pH of surrounding environment, thickness of MTA filling and dentinal wall can affect sealing ability of MTA33,60-61.

6. EFFECT OF BISMUTH OXIDE ON PHYSICAL PROPERTIES OF MTA

Radiopacity of a root end filling material is one of its basic requirements50. Bismuth oxide exhibit higher absorption of shorter wavelength X-ray radiations8 and due to its presence, MTA possess a radiopacity of 7.17 mm equivalent thickness of aluminum4. Torabinejad and White added 20% wt bismuth oxide in MTA to render it radiopaque. Both GMTA and WMTA consist of 75% wt. calcium silicate, 20% wt. bismuth oxide and 5% wt. calcium sulphate10. MTA contains 10-30 µm sized particles of bismuth oxide20 which are trapped in the amorphous phases of set GMTA and WMTA13. Several studies have evaluated the effect of bismuth oxide on properties of MTA. Kim et al. evaluated the effect of bismuth oxide addition on radiopacity and cytotoxicity of Portland cement and reported a linear correlation between the observed radiopacity and amount of bismuth oxide, whereas no difference was found in the amount of bismuth oxide and cytotoxicity of the material62.
Coutinho-Filho et al. evaluated MTA, PC and a combination of PC and bismuth oxide and revealed positive correlation between radiopacity and concentration of bismuth oxide. The outcomes of histological examination after subcutaneous implantation in rats suggested that all tested materials were biocompatible63 .
However, addition of bismuth oxide can affect physical and mechanical properties of MTA and reduce compressive strength by incorporating flaws and increased porosity in the set structure7,21,45. Bismuth oxide is simultaneously present in the set structure of MTA as unreacted filler and a part of C-S-H20. Darvell and Wu considered that bismuth oxide act as inert filler with no contribution in setting reaction of MTA21. Whereas, Camilleri reported relatively lower peaks for bismuth oxide in XRD pattern of hydrated MTA compared to unhydrated MTA and interpreted the lower peak heights to “use up” of bismuth in hydration mechanism and suggested that bismuth takes active part in setting reaction and replaces silica in C-S-H64.

7. CONCLUSIONS

This review systematically summarized the contemporary knowledge of MTA with respect to materials science and clinical dentistry. The last 15 years have seen major developments in the chemistry of MTA due to its potential use in dentistry and biocompatibility. The nature of setting reactions favors placement in endodontic procedures having moist environment as it consists of fine hydrophilic particles of natural minerals. MTA facilitates the formation of HA like mineral structure on its surface when it comes in contact with tissue fluids hence provide a biological seal. Hydration of MTA forms a colloidal gel that solidifies in due course however, the physical properties are influenced by various factors therefore, different results may be obtained during investigation of MTA’s physical properties.

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  1. M.Phil Student, Department of Dental Materials, Army Medical College, National University of Sciences and Technology, Islamabad, Pakistan. Department of Operative Dentistry, Bolan Medical College, University of Balochistan, Quetta, Pakistan.
  2. Associate Professor, Department of Dental Materials Science, FMH College of Medicine and Dentistry, University of Health Sciences, Lahore, Pakistan
  3. Assistant Professor, Department of Dental Materials, Army Medical College, National University of Sciences and Technology, Islamabad, Pakistan.
  4. Professor, Department of Dental Materials Science, Baqai Dental College, Baqai Medical University, Karachi, Pakistan.
    Corresponding author: “Dr Muhammad Amber Fareed ”<docamberfareed@hotmail.com >