Basic Research—Technology

Particle Size Changes in Unsealed Mineral TrioxideAggregate PowderWilliam N. Ha, BDSc, Bill Kahler, PhD, DClinDent, and Laurence James Walsh, DDSc, PhD

Abstract

Introduction: Mineral trioxide aggregate (MTA) iscommonly supplied in 1-g packages of powderthat are used by some clinicians across several treat-ments against the manufacturer’s instructions. ProRootMTA cannot be resealed after opening, whereas MTAAngelus has a resealable lid. This study assessedchanges in particle size distribution once the packaginghad been opened. Methods: Fresh ProRoot MTA andMTA Angelus powder were analyzed by using laserdiffraction and scanning electron microscopy andcompared with powder from packages that had beenopened once and kept in storage for 2 years. TheProRoot packet was folded over, whereas the MTAAngelus jar had the lid twisted back to its original posi-tion. Results: After 2 years, ProRoot MTA powdershowed a 6-fold increase in particle size (lower 10%from 1.13 to 4.37 mm, median particle size from 1.99to 12.87 mm, and upper 10% from 4.30 to 34.67 mm),with an accompanying 50-fold change in particle surfacearea. MTA Angelus showed only a 2-fold increase inparticle size (4.15 to 8.32 mm, 12.72 to 23.79 mm,and 42.66 to 47.91 mm, respectively) and a 2-foldchange in particle size surface area. Conclusions:MTA reacts with atmospheric moisture, causing an in-crease in particle size that may adversely affect the prop-erties and shelf life of the material. Smaller particleshave a greater predisposition to absorb moisture.Single-use systems are advised. (J Endod2014;40:423–426)

Key WordsLaser diffraction, mineral trioxide aggregate, particlesize, pre-hydration, single-use applications

From the University of Queensland School of Dentistry, Bris-bane, Queensland, Australia.

Address requests for reprints to Dr Laurence James Walsh,University of Queensland School of Dentistry, 200 Turbot Street,Brisbane, QLD 4000, Australia. E-mail address: l.walsh@uq.edu.au0099-2399/$ - see front matter

Copyright ª 2014 American Association of Endodontists.http://dx.doi.org/10.1016/j.joen.2013.10.018

JOE — Volume 40, Number 3, March 2014

Mineral trioxide aggregate (MTA) is an important endodontic material withmultipleuses (1–3). The MTA patent describes its ideal composition as 1 part bismuth

oxide and 4 parts Portland cement (4). The latter component is hygroscopic andcan absorb atmospheric moisture. ProRoot MTA (MTA-P) (Dentsply Maillefer,Ballaigues, Switzerland) is supplied in 1-g packets, with the instructions ‘‘1 gram-1treatment.’’ Therefore, the packet should not be opened until its use, and any powdernot dispensed for the patient should not be reused. Nevertheless, a search of the termMTA Uses on a popular Internet discussion forum, ‘‘Dentaltown,’’ revealed numerouspostings from clinicians who are usingMTA-P packets formultiple applications to lowerthe cost per application. The high cost influences its clinical use. A recent survey hasshown that if cost was not an issue, some 85% of pediatric dentists and endodontistswould use it over formocresol (5). Furthermore, cost has limited the uptake of MTAby educational institutions (6, 7).

Whereas opened packets of MTA-P cannot easily be resealed, the container forwhite MTA Angelus (MTA-A) (Angelus Soluc~oes Odontol�ogicas, Londrina, Brazil)has a resealable lid. The packaging is marked with the international standard symbolfor ‘‘Do not reuse, Single use only, Use only once’’ (8, 9); however, it is marketed asproviding 7 applications for 1 gram (10).

The existing literature does not address whether multiple use of MTA from thesame container affects its properties. It can be expected that because MTA powder ishygroscopic, when it is left exposed to atmospheric moisture, it will react in a similarway as MTA powder mixed with water (1). The particles will begin to hydrate andagglomerate with neighboring particles into larger structures. Therefore, MTA powderthat has had significant exposure to moisture should show an increased particle size. Alarger particle would have a lower surface area than that of the particles from which itwas formed and thus be less reactive, which could have implications for setting time,compressive strength, and alkalinity.

Although the thermodynamic process involved in cement hydration is notcompletely understood (11), a simplified mathematical model to understand therelationship between particle size and the degree of hydration is the following: (12)

aðrÞ ¼ 1��1�

�kt

r

��3

(Equation 1)

Alpha (a) is the degree of hydration, t is the time, k is the rate constant, and r is theradius of the particle. From this formula, there is an exponential increase in the degreeof hydration as the particle size is reduced. This also suggests any differences in particlesize distribution between MTA-P and MTA-A may also alter the degree of hydration.

Studies on the particle size distribution of MTA are few (13–15). The patent forwhite MTA describes the Portland cement component as having 90% of the particlesfiner than 25 mm, 50% of the particles finer than 9 mm, and 10% of the particlesfiner than 3 mm (16). Bismuth oxide powder is supplied in various particle sizedistributions, and the patent does not discuss the resultant particle size distributiononce bismuth oxide has been added. Scanning electron microscopic (SEM) examina-tion of MTA indicates that particles range from <1 mm to as large as 50 mm (17, 18).

The distribution of MTA particle size has been determined by using a flow par-ticle analyzer; however, this method cannot accurately measure particles that are lessthan 1.5 mm or greater than 40 mm (17, 18). An alternative method for assessingparticle size distribution is laser diffraction, which measures the size of particles

MTA Particle Size Changes over Time 423

TABLE 1. Particle Size Distribution of MTA

Cement typeD10 (10% of particles are below

this size) (mm)D50 (median particle

size) (mm)D90 (90% of particles are below

this size) (mm)

MTA-P freshly opened 1.13 1.99 4.30MTA-P aged 2 y 4.37 12.87 34.67MTA-A freshly opened 4.15 12.72 42.66MTA-A aged 2 y 8.32 23.79 47.91Latex 0.8 mm 0.69 0.77 0.86Latex 1 mm 0.81 0.91 1.03Latex 3 mm 2.73 2.88 3.06Latex 5 mm 4.66 5.59 8.18Latex 20 mm 15.22 18.55 21.05

Basic Research—Technology

through the scattering of the laser beam through a dispersedparticulate sample. The MicroPlus analyzer (Malvern Instruments,Worcestershire, UK) can accurately measure particles from 0.05–550 mm and is widely used in the cement industry for qualitycontrol of Portland cement (19).

This study was undertaken to evaluate changes in the particle sizeof MTA over time that result from atmospheric exposure during storage.It sets the groundwork for future work to explore concerns with MTAproperties when taken from multiple-use containers.

Materials and MethodsThis study replicated the methods of an investigation of fly ash, an

industrial cementitious product similar to MTA (20, 21). For laserdiffraction analysis, the refractive index used for MTA was 1.842,which was calculated as a weighted average from the refractive indexof 20% bismuth oxide and 80% Portland cement.

Figure 1. Particle size distribution of MTA-P and MTA-A when fresh and 2years after having the packaging opened.

424 Ha et al.

A packet of MTA-P (lot 09001921) was opened, folded on itself,and kept within closed boxed packaging for 2 years under normalroom conditions, and another unopened packet of the same lot servedas the control. A jar of MTA-A (lot 12862) was opened once, reclosed,and then kept for 2 years. For comparison, an unopened jar of freshmaterial (lot 21381) was tested soon after receipt from the supplier.The manufacturer’s instructions for MTA-P suggest storing MTAbetween 10�C and 25�C. All containers were kept at room temperaturein a cabinet away from sunlight, in accordance with the manufacturer’sinstructions for storage, for a period of 2 years. This duration was basedon the expiration date being 3 years from manufacture (S. Freeman,personal communication). The study was performed in Brisbane, acity with a subtropical climate, with a mean maximum temperature of25.3�C and a mean minimum temperature of 15.5�C (22).

One gram of each MTA sample was placed into 1 L distilled waterwithin a Malvern MicroPlus analyzer, with analysis taking 4 seconds.The water dispersant was under continuous ultrasonic agitation toprevent agglomerated cement sinking to the bottom of the beakerand to prevent reagglomeration. The water was supplemented with1 g/L sodium hexametaphosphate (Calgon) to prevent the MTA powderfloating on the surface of the water and also to prevent hydration of thecement during the analysis. The period from placing the powder into thedispersant until analysis was less than 10 seconds. Spherical latex beadsof 5 known sizes (0.8–20 mm) (lot #011899; ProSciTech Pty Ltd,Kirwan, Australia) served as controls for instrument calibration. Parti-cle area was estimated by using spherical and cubic particlemodels withmean particle size as the diameter.

Samples of fresh and 2-year-aged MTA-P were examined by usingSEM with backscatter imaging. Powder was sprinkled onto carbon tapeand left uncoated. Images were taken at 15.0 kV with final magnificationof �1200 under low vacuum conditions by using an FEI 200 SEM(Quanta, Hillsboro, OR).

ResultsResults from laser diffraction analysis are presented in Table 1 and

Figure 1. MTA-P underwent a 6.5-fold increase in median particle sizefrom 1.99 to 12.87 mm, whereas MTA-A showed a 2-fold increase in itsD10 and D50, although its D90 remain relatively unchanged.

In terms of particle surface area, for MTA-P the increase in particlesize from fresh to aged product was from 15.2 to 725.4 square microns,a 47.7-fold change in available surface area. For MTA-A, the surfacearea change was from936.5 to 1882.6 squaremicrons, a 2-fold change.When particles were modeled as being perfectly cubic, the surface areareduction was 6.9-fold for MTA-P and 1.4-fold for MTA-A.

Backscattered SEM images of MTA revealed the agglomeration ofparticles (Fig. 2). In these images, bismuth oxide appears as brightwhite particles (approximately 1–4mm), which corresponds to the firstmodal peak of 3–4 mm seen in the particle size distribution for MTA-P.

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Figure 2. Backscatter-mode SEM imaging of MTA-P. Bismuth oxide appears as white particles because of the higher atomic number of bismuth. (A) Fresh MTA;(B) MTA after 2 years after opening packaging.

Basic Research—Technology

DiscussionThis study has identified differences in particle size distribution

between MTA-P and MTA-A and between fresh and aged samples.The instrumentation used had superior measurement abilitiescompared with that used in past studies (13, 14), and its accuracywas verified by using latex spheres. Changes over time with the2 different MTA products likely reflect differences in their particlesize (as per Equation 1) and their packaging systems, with MTA-P hav-ing smaller particles at baseline and thus greater surface area, as well asbeing stored in a non-resealable packet. The combination of thesefactors results in MTA-P particles showing more dramatic agglomera-tion over time.

A potential technical limitation of the study relates to the dispersalmethod used for samples. Ethanol is generally preferred over water asthe dispersant for cements because it eliminates the confounding effectof reactions with water. Samples were dispersed in water rather thanethanol for occupational health and safety reasons, and sodium hexam-etaphosphate was included to retard hydration, an approach that isaccepted in the cement industry for particle analysis (23) (B. Collins,e-mail to William Ha, December 12, 2012). The Malvern analyzer takesonly 4 seconds to analyze the whole 1-g amount of the sample, a timeperiod during which only limited setting would occur, even if sodiumhexametaphosphate was not used (24) (B. Collins, e-mail to WilliamHa, December 12, 2012). An additional limitation is that the test andcontrol samples of MTA-A were not from the same batch, which relatesto how the material was supplied at the time the study was commenced.

The increase in MTA particle size during storage alters the particlesize distributions, with the smaller particles being the most reactive. Forexample, with MTA-A, changes in D50 and D10 are more prominentthan D90, which would be predicted from Equation 1. Any MTA productwith an even smaller median particle size than MTA-P would be evenmore prone to effects of atmospheric moisture during packaging andstorage. This would require heightened attention to the single-use prin-ciple as well as to the quality of the seal during storage and may warrantthe inclusion of a desiccant to ensure the shelf life of a sealed containerremains adequate.

A concern with partial or pre-hydration of particles is that largerparticles of reacted cement impair mixing (25) and would likely retardthe setting of the Portland cement component as well as decrease itscompressive strength once set (25, 26). Past studies have revealedthat pre-hydration of MTA can cause the material to fail to set (27).

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The particle size distribution graphs show a bimodal distribution,which results from the combination of gaussian distributions of thesmaller bismuth oxide powder particles (3.5 mm in MTA-P and 8mm in MTA-A) with Portland cement particles that are much larger.Bismuth oxide powder is insoluble in water and is believed to be chem-ically inert (28). There was no evidence of the bismuth oxide particlesreacting with moisture over time in the study.

In terms of clinical implications, MTA-A in a bottle was betterable to resist pre-hydration than MTA-P in a packet when openedonce during the 2-year period. MTA-A product advertising states that1 g is sufficient for 7 applications (10). If an MTA-A bottle was opened7 times, one would expect much greater changes than observed in thestudy. It is suggested that MTA packaging where multiple uses arepromoted should feature a desiccant. An example of this is MTA Plus(Avalon Biomed, Bradenton, FL), which is packaged as 8 g MTA powderin a desiccant-lined container. There is as yet no published evidence toillustrate that a desiccant canmaintain the stability of MTA overmultiple-use conditions. Placing cement that has pre-hydrated because ofexposure to atmospheric moisture in a drying environment will not re-turn the cement to the same state before pre-hydration (26). Therefore,any product that is not single use has the potential for hydration on everyinstance when exposed to the atmosphere.

ConclusionMTA undergoes an increase in particle size once themanufactured

seal has been broken. The increase in particle size from exposure toatmospheric moisture differs between brands and may affect thehandling and subsequent clinical performance of the material, a pointthat requires further investigation. In the absence of this information,the material should be handled as a one-use application as recommen-ded by the manufacturer.

AcknowledgmentsThe authors thank Gunz Dental and Dentsply for providing

materials for the study. The expert assistance of Dr Lei Chai withSEM imaging andMrMichael Archer, General Manager of SiPowders,for assistance in particle size analysis, is gratefully acknowledged.

Supported by a grant from the Australian Dental ResearchFoundation.

The authors deny any conflicts of interest related to this study.

MTA Particle Size Changes over Time 425

Basic Research—Technology

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