The constitution of mineral trioxide aggregate

The constitution of mineral trioxide aggregate

Dental Materials (2005) 21, 297–303 www.intl.elsevierhealth.com/journals/dema The constitution of mineral trioxide aggregate Josette Camilleria, Fra...

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Dental Materials (2005) 21, 297–303

www.intl.elsevierhealth.com/journals/dema

The constitution of mineral trioxide aggregate Josette Camilleria, Franco E. Montesinb, Ken Bradyc, Richard Sweeneyd, Richard V. Curtisa, Thomas R. Pitt Forda,* a

Department of Conservative Dentistry and Dental Biomaterials, Guy’s, King’s and St Thomas Dental Institute, King’s College London, Guy’s Hospital, London SE1 9RT, UK b Faculty of Architecture and Civil Engineering, University of Malta, Malta c Electron Microscopy Unit, School of Biomedical Sciences, King’s College London, London, UK d Department of Materials, Imperial College, London, UK Received 6 November 2003; received in revised form 6 April 2004; accepted 4 May 2004

KEYWORDS Constitution; Mineral trioxide aggregate; Portland cement

Summary Objectives: The aim of this study was to determine the constitution of a commercially available root-end filling material, mineral trioxide aggregate, (MTA) (ProRoot MTA, Tulsa Dental, Tulsa, OK, USA). The surface morphology of the material with various treatment conditions was also evaluated. Methods: The constitution of two commercial versions of MTA was determined before and after mixing with water. The unset material was analysed using Energy Dispersive Analysis by X-ray (EDAX) in a scanning electron microscope (SEM) and X-ray diffraction (XRD). The first technique identified the constituent elements while XRD analysis identified the compounds or phases present. The set material was evaluated using EDAX. The surface morphology of the material stored under various conditions (100% humidity, immersion in water, or immersion in phosphate solution) was evaluated using SEM. Results: The EDAX showed the white MTA to be composed primarily of calcium, silicon, bismuth and oxygen, with the gray MTA also having small peaks for iron and aluminum. The XRD analysis showed gray MTA to be composed primarily of tricalcium silicate and dicalcium silicate. The surface morphology of the materials differed under the various conditions, particularly following immersion in phosphate solution with crystal formation. Significance: The commercial versions of MTA were shown to have broadly similar constitution to ordinary Portland cement except for the addition of bismuth compounds. The white MTA did not contain iron. Q 2004 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Introduction

* Corresponding author. Tel.: C44 20 7188 1162; fax: C44 20 7188 1159. E-mail address: [email protected] (T.R.P. Ford).

Mineral trioxide aggregate (MTA) was developed at Loma Linda University for use as a root-end filling material in surgical endodontic treatment [1]. It has been patented [2], has received the approval of the

0109-5641/$ - see front matter Q 2004 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2004.05.010

298 Federal Drug Administration (FDA) in the USA and is commercially available as ProRoot MTA (Tulsa Dental Products, Tulsa, OK, USA). Initially a gray version was produced but more recently a white version has become available. The MTA powder consists of fine hydrophilic particles. The principal compounds reported to be present in this material are tricalcium silicate, tricalcium aluminate, tricalcium oxide, and silicate oxide [1]. In that study, X-ray analysis revealed the presence of two phases: the crystalline material was essentially calcium oxide and amorphous calcium phosphate. Hydration of MTA powder resulted in the formation of a colloidal gel that hardened. The pH of MTA immediately after mixing was 10.2, rising to 12.5 after 3 h [1]. The purpose of this study was to determine the constitution of two commercial versions of MTA and to analyse the surface morphology of the powder, and the set material under various conditions.

Materials and methods Analysis of powders The constitution of two versions of MTA, white and gray, (ProRoot MTA Tulsa Dental Products) was determined using Energy Dispersive Analysis by Xray (EDAX) in a scanning electron microscope (SEM) (Hitachi S3500, Hitachi, Wokingham, UK). A thin layer of powder was dispersed over a polymethylmethacrylate slab mounted on an aluminum stub. The specimens were carbon coated (K250, Emitech, Ashford, UK) for electrical conductivity. The specimens were then viewed under the SEM, and EDAX was used to determine the constituent elements of the powders. Secondary electron and back-scattered images were also taken at up to !1000 magnification. Two stubs were made for each material and the analysis was performed twice for each sample. In addition, phase analysis was carried out using X-ray diffraction (XRD) in an automated powder diffractometer (Philips PW 1700, Eindhoven, The Netherlands) using Cu Ka radiation and a secondary crystal monochromator. Samples were presented in powder form on a single crystal sample holder, which avoids unwanted diffraction peaks. Phase identification was accomplished by use of searchmatch software utilizing the International Center for Diffraction Data (ICDD) database (International Center for Diffraction Data, Newtown Square, PA, USA).

J. Camilleri et al.

Analysis of set cements Ordinary Portland cement (Central Cement, Italy) was investigated as well as MTA; 1 g of gray MTA or Portland cement was mixed with 0.35 ml of distilled water to produce a homogeneous paste. The materials were compacted between glass plates in brass molds, 5 mm in diameter, and were allowed to cure in the mold at 100% humidity until the cements had set. Removal from the molds was performed 6 h after mixing, following which all the specimens were placed in 30 mm tissue culture dishes and kept at 100% humidity for 3 days at 37 8C, when they were treated in one of the following ways: 1. Maintained at 100% humidity at 37 8C (control). 2. Immersed in water for 4 h followed either by air drying for 12 h or critical-point drying for 1 h (Emitech K850). 3. Immersed in 2.5% glutaraldehyde in 0.2 M phosphate solution for 4 h, after which the solution was replaced with a glutaraldehyde wash solution for 24 h. The samples were then desiccated with ascending grades of ethanol, and were then either air-dried or critical-point dried. This procedure was used to mimic how specimens would be treated if subjected to cell culture. The specimens were then mounted directly on aluminum stubs with carbon cement (Leit-C, Emitech), and were carbon coated for electrical conductivity as before. The specimens were then viewed under the SEM. Secondary electron imaging was performed for the surface analysis of the materials, and EDAX was undertaken for chemical analysis; three samples were made for each material for surface morphological evaluation and analysis was performed twice for each sample in different areas.

Results Powder analysis Scanning electron micrographs of gray and white MTA gave differing images. Secondary electron images of white MTA showed the presence of small irregular particles interspersed with some elongated needle-like particles (Fig. 1a). Backscattered electron images of gray MTA showed the material to be composed of small irregular particles with some much larger particles as well as elongated particles (Fig. 1b).

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Figure 1 (a) Secondary electron image of white MTA showing small irregular particles interspersed with some elongated needle-like particles (!350). (b) Back-scatter electron image of gray MTA showing small irregular particles with some much larger particles as well as elongated particles (!350).

The EDAX for white MTA showed the presence of calcium, silicon, bismuth and oxygen (Fig. 2a). The EDAX analysis of gray MTA (Fig. 2b) showed the material to be composed of calcium, silicon, aluminum, iron, bismuth and oxygen. Calcium and silicon were the predominant elements. White MTA was composed primarily of tricalcium silicate and bismuth oxide (Fig. 3a) while gray MTA was composed primarily of tricalcium silicate, dicalcium silicate and bismuth oxide (Fig. 3b); minor phases were not identified. Comparison of the XRD analysis of both versions of MTA showed

the materials to have similar phase constitutions (Fig. 4). Scanning electron micrographs of Portland cement showed the material to be very similar to gray MTA, except for the absence of smooth elongated particles.

Analysis of set cements Both set MTA and Portland cement showed a smooth surface morphology following immersion in water (Fig. 5a). The presence of crystals was only

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Figure 2 (a) EDAX of white MTA revealing the presence of calcium, silicon, bismuth and oxygen; (b) EDAX of gray MTA revealing the presence of calcium, silicon, aluminum, iron, bismuth and oxygen.

observed with both MTA and Portland cement after the set materials had been placed in the phosphate solution (Fig. 5b). The EDAX showed the presence of phosphorus on both MTA and Portland cement after being placed in the phosphate solution (Fig. 6).

Discussion Analysis of the constitution of the white and gray versions of MTA has shown that both materials are

similar to Portland cement, but with bismuth oxide added, presumably to make the materials radiopaque for dental use. The white MTA has been created by the exclusion of iron compounds rather than by the addition of other elements. It did not contain such large particles as gray MTA, and that would account for its improved clinical handling properties. The MTA was observed to be purely crystalline as verified by XRD. Previously a fluorescence spectrometer was used to compare two prototype variants of MTA with Portland cement; similar chemical constituents of these

Constitution of mineral trioxide aggregate

Figure 3

(a) XRD analysis of white MTA; (b) XRD analysis of gray MTA indicating major phases present.

materials were reported [3]. The material is derived primarily from calcium oxide 50–75%, silicon dioxide 15–25% and aluminum oxide [2]. These raw materials are clinkered in a kiln to

Figure 4

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produce tricalcium silicate, dicalcium silicate and tricalcium aluminate; a small amount of tetracalcium aluminoferrite is produced from the iron impurities in the ore. On addition of water these

XRD patterns of white and gray MTA indicating similar phase constitution of the materials.

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Figure 5 (a) Scanning electron micrograph of MTA immersed in water (!350); (b) MTA immersed in phosphate solution and critical-point dried showing the presence of crystals on the surface (!50).

compounds react to produce calcium silicate hydrate gel. The presence of phosphorus compounds was not observed in the analysis of the unset material; this was in contrast to earlier reports of analysis of set cements [1,4]. Phosphorus was observed in the analysis of the set cements in this study but only when the materials had been immersed in a phosphate solution. This indicates that the phosphorus compound was not an integral part of the material but derived from the subsequent treatment of the set cements. The phosphate solutions are used to fix cells in biocompatibility assessment, as in earlier studies [1,4].

Calcium hydroxide produced as a reaction product on the surface of Portland cement may react with carbon dioxide present in air to form calcium carbonate. The carbonation could be accentuated by critical-point drying, which utilizes carbon dioxide in the drying of the specimens, as reported previously [5]. The absence of crystals in the samples allowed to set in water and then criticalpoint dried would imply that the crystals observed on the specimens subjected to the phosphate solution were unlikely to be carbonation products. The presence of bismuth oxide had only been reported in one previous study [6]. The absence of bismuth oxide in other analyses would have been due

Constitution of mineral trioxide aggregate

Figure 6

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EDAX of MTA immersed in the phosphate solution showing the phosphorus peak.

to the use of prototype versions of MTA [1,3,4], which did not contain any radiopacifying agent in contrast to using the commercial version of MTA in this study. The smooth elongated particles present in gray MTA powder were considered to be bismuth oxide. The EDAX was performed with the specimens placed directly on aluminum stubs and held in place with carbon cement because placing the specimens on glass cover-slips could detect the elemental constitution of the cover-slips. The XRD showed gray MTA to be composed primarily of tricalcium silicate and dicalcium silicate; it is likely that small amounts of tricalcium aluminate and tetracalcium aluminoferrite were present. These compounds are formed during manufacture from calcium oxide, silicon dioxide, aluminum oxide and iron oxide. No iron was found in the white MTA; this mirrors the constitution of white Portland cement. Because the constituents of MTA are similar to those of Portland cement, the setting reaction is also expected to be similar. The principal constituents, tricalcium silicate and dicalcium silicate, react with water to produce a poorly-crystallized hydrated salt and calcium hydroxide [7]. The authors’ interpretation is that set MTA can be considered as calcium hydroxide contained in a silicate matrix. The presence of calcium hydroxide accounts for the cement being highly alkaline (pH 12.5) [1]; this may account for its biocompatibility [3–5]. Both white and gray variants of MTA have been shown to have a similar chemical constitution to that of Portland cement except for the addition of bismuth compounds.

Acknowledgements The following are thanked: Association of Commonwealth Universities for funding; Faculty of Architecture and Civil Engineering, University of Malta and the Department of Materials, Imperial College London for access to equipment; Ms Fiona Winning for expert help in preparing the samples and Mrs Diane Brook for her encouragement during the period of study.

References [1] Torabinejad M, Hong CU, McDonald F, Pitt Ford TR. Physical and chemical properties of a new root-end filling material. J Endodont 1995;21:349–53. [2] Torabinejad M, White DJ, Tooth filling material and use. US Patent Number, 5,769,638; May 1995. [3] Mitchell PJC, Pitt Ford TR, Torabinejad M, McDonald F. Osteoblast biocompatibility of mineral trioxide aggregate. Biomaterials 1999;20:167–73. [4] Koh ET, Torabinejad M, Pitt Ford TR, Brady K, McDonald F. Mineral trioxide aggregate stimulates a biological response in human osteoblasts. J Biomed Mater Res 1997;37:432–9. [5] Abdullah D, Pitt Ford TR, Papaioannou S, Nicholson J, McDonald F. An evaluation of accelerated Portland cement as a restorative material. Biomaterials 2002;23:4001–10. [6] Estrela C, Bammann LL, Estrela CR, Silva RS, Pecora JD. Antimicrobial and chemical study of MTA, Portland cement, calcium hydroxide paste, Sealapex and Dycal. Braz Dent J 2000;11:3–9. [7] Lea FM. The chemistry of cement and concrete, 3rd ed. London: Edward Arnold; 1970 p. 177–86.