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Mercury Intrusion Porosimetry and Assessment of Cement-dentin Interface of Anti–washout-type Mineral Trioxide Aggregate L.M. Formosa,* D. Damidot,†‡ and Josette Camilleri, PhD§ Abstract Introduction: One of the disadvantages of mineral trioxide aggregate (MTA) is washout (ie, the tendency of freshly prepared cement paste to disintegrate upon early contact with physiological fluids). A novel MTA (MTA Plus; Prevest Denpro, Jammu City, India) exhibits low washout and superior physical properties when mixed with a gel instead of water. When used as a root-end filler, MTA is in contact with both bone and root dentin. This study aimed to investigate the porosity and interfacial characteristics of the novel MTA mixed with water or antiwashout gel. Methods: Porosity was evaluated after 1 or 28 days of immersion in Hank’s balanced salt solution using mercury intrusion porosimetry. The root dentin to material interface was investigated using a scanning electron microscope and energy-dispersive X-ray spectroscopy complete with line scans and elemental maps. Results: Anti– washout-type MTA Plus was found to have lower initial porosity than MTA Plus mixed with water although this trend was reversed after 28 days of immersion in physiological fluid. Both materials exhibited good marginal adaptation. The diffusion of silicon, calcium, and phosphorus across the cement/dentin interface was observed. Conclusions: MTA Plus mixed with antiwashout gel was found to have lower initial porosity than MTA Plus mixed with water. Both materials exhibited good marginal adaptation and the diffusion of silicon, calcium, and phosphorous across the cement/dentin interface. Thus, the anti–washout-type MTA can be considered to be a suitable substitute for ordinary MTA in all its indications. (J Endod 2014;-:1–6)
Key Words Antiwashout, dentin-material interface, mercury intrusion porosimetry, mineral trioxide aggregate, root-end filling materials
W
hite mineral trioxide aggregate (MTA) is a dental cement with an ability to set in the presence of moisture (1). MTA has numerous applications including pulp capping (2), apexification (3), repair of root perforations (4), root-end filling (5), and others (6). When MTA is used as a root-end filling material, it is in contact with both dentin and bone at the periapex. The interaction of MTA with physiological solutions has been well documented. The Portland cement component of MTA interacts with physiological fluids, and a reaction of calcium hydroxide with phosphates present in tissue fluids occurs (7–9). An amorphous calcium phosphate phase is initially formed and then transformed to an apatite phase, with the latter consisting of calcium-deficient, poorly crystalline, B-type carbonated apatite crystallites. Amorphous calcium phosphate is a key intermediate that precedes biological apatite formation in skeletal calcification (9). The interaction of MTA with dentin also has been documented more recently. The prolonged contact of mineralized dentin with calcium silicate–based cements has an adverse effect on the integrity of the dentin collagen matrix. The amount of collagen extracted was limited to the contact surface (10). Both MTA and Biodentine (Septodont, Saint Maur-des-Fosses, France) caused an alteration of strength and stiffness of dentin (11). Furthermore, calcium silicate–based materials can lead to dentin remineralization in the presence of synthetic tissue fluid (12). One of the drawbacks of MTA when used as a root-end filling material is its poor resistance to washout (13) (ie, the tendency of freshly prepared cement paste to disintegrate upon premature contact with blood or other fluids) (14). Washout of a root-end filling material can occur when rinsing an osteotomy site (13), resulting in a compromised root-end seal. A novel mineral trioxide aggregate (MTA Plus; Prevest Denpro, Jammu City, India) that has a finer particle size than the MTAs currently available for clinical use (15) is supplied with either water or a water-based antiwashout gel intended to be mixed with the MTA powder in place of water to improve the washout resistance. The antiwashout gel is a water-based polymer that when mixed with MTA Plus powder improves the material workability and handling (15). This gel has already been shown to markedly improve the washout resistance of MTA (16). The use of the antiwashout gel together with the finer particle size of the MTA powder results in a material with improved physical and chemical properties. MTA Plus mixed with antiwashout gel exhibited reduced fluid uptake in the early stages of reaction, a reduced setting time, and enhanced compressive strength when compared with the same formulation mixed with water (15). However, the push-out bond strength of MTA Plus mixed with gel was lower than that of the material mixed with water (17). The improvement in the physical properties of Portland cement–based materials is usually related to reductions in the water/cement ratio (18) and the additions of
From the *Department of Metallurgy and Materials Engineering, Faculty of Engineering, University of Malta, Malta; †University of Lille Nord de France, Lille, France; EM Douai, Douai, France; and §Department of Restorative Dentistry, Faculty of Dental Surgery, University of Malta, Malta. Address requests for reprints to Dr Josette Camilleri, Department of Restorative Dentistry, Faculty of Dental Surgery, University of Malta, Medical School, Mater Dei Hospital, Msida MSD 2090, Malta. E-mail address: [email protected] 0099-2399/$ - see front matter Copyright ª 2014 American Association of Endodontists. http://dx.doi.org/10.1016/j.joen.2013.11.015
‡
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Basic Research—Technology polymers that allow a reduction in the water/cement ratio without modifications to the material workability (19). The low water to cement ratio results in a reduction in material porosity. The purpose of this study was to investigate the porosity and interface characteristics of MTA Plus mixed with antiwashout gel (MTA-AW) and to compare these properties with MTA Plus mixed with water, which served as the control (MTA-W).
Materials and Methods The materials used in this study included MTA Plus (compounded by Prevest Denpro, Jammu, India, for Avalon Biomed Inc, Bradenton, FL) lot #2011022801 mixed with either distilled water at a water to powder ratio of 0.35 (MTA-W) or mixed with antiwashout gel (compounded by Prevest Denpro, Jammu, India, for Avalon Biomed Inc) (MTA-AW). The gel was dosed by weight (0.350 g gel per gram MTA Plus powder). The testing was performed on days 1 and 28, and the materials were stored immersed in Hank’s balanced salt solution (HBSS [H6648; Sigma-Aldrich, St Louis, MO]).
Mercury Intrusion Porosimetry Cube specimens (7 7 7 mm) were cast and immersed in HBSS for 1 or 28 days; after this, they were taken out of the solution and dried in a desiccator in the presence of soda lime to avoid surface carbonation. The porosity was measured in a 2-stage process using a mercury intrusion porosimeter (Micromeritics AutoPore IV; Micromeritics France SA, Paris, France). For the first (low pressure) stage, the specimens (of approximate volume 0.343 cm3) were weighed on an analytic balance, transferred into a glass penetrometer cell with a total internal volume of 15.52 cm3 (of which 0.366 cm3 was the stem volume), and the entire assembly was weighed. Precise measurement of the bulk volume of each sample was then performed using a calibrated mercury displacement pycnometer within the porosimeter’s low-pressure chamber. The chamber was evacuated to a pressure of 50 mm Hg over an evacuation time of 300 seconds, and mercury was introduced to the penetrometer at a filling pressure of 0.45 psi. The filled penetrometer was then weighed on an analytic balance and transferred to the high-pressure cell. The pressure was subsequently increased in 60 pressure increments to a maximum pressure of 33,000 psi (227.53 MPa). The pressure increments were chosen to correspond to even intervals on a logarithmic scale. After each pressure increment, the machine was set to pause for 20 seconds to allow the pressure and volume readings to stabilize before recording a pressure/volume data pair. The first stage of testing involved filling the cup and capillary stem with mercury. The pressure on the filled penetrometer was increased, resulting in mercury intrusion into the pores of the test sample and a corresponding decrease of the mercury level in the stem. A capacitance change between the mercury column inside the stem and the metal cladding on the outer surface of the stem allowed the volume of the mercury intruded in the test specimen to be measured to a high degree of accuracy. The pressure was then decreased back down to atmospheric pressure in 25 increments, measuring extrusion volume at each step. Porosimetry data were processed using the software supplied with the machine (Micromeritics France SA, Paris, France), which generates volume and size distributions from the pressure versus intrusion data using a method based on the following Washburn 4 equation: D ¼ 4g cos P , where D is the pore diameter, g is the surface tension of mercury (485 dynes/cm), P is the applied pressure, and F is the contact angle (130 ) between the mercury and the sample. 2
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The average pore diameter (4V/A) was given directly by the porosimeter. It was assumed that all pores are cylindrical in shape. Hence, by dividing the pore volume (V = pd2L/4) by the pore area (A = pdL), the average pore diameter (d) was determined (equal to 4V/A) (20). The bulk and skeletal densities were calculated. Bulk density is the density of the cement specimens as a whole, which includes the volume occupied by the pores. Skeletal density represents the density of the cement ‘‘skeleton’’ without the pores. The total percentage porosity was calculated from the difference between the bulk and skeletal densities.
Assessment of Tooth to Material Interface Single-rooted teeth extracted for periodontal reasons were used. The teeth were decoronated, and the root length was standardized to 15 mm. The root canals were instrumented with ProTaper rotary nickel-titanium instruments (Dentsply Maillefer, Ballaigues, Switzerland) and irrigated with 10 mL 5% sodium hypochlorite followed by 2 mL EDTA to remove the smear layer. The canals were dried and obturated with gutta-percha and AH Plus sealer (Dentsply Maillefer) using the warm vertical condensation technique (System B; SybronEndo, Orange, CA). The coronal access was restored with glass ionomer cement (Fuji IX; GC Europe, Leuven, Belgium). The root ends were resected and root-end preparations were prepared to a depth of 3 mm using an ultrasonic tip (KiS-ID; Obtura-Spartan, Earth City, MO) attached to an ultrasonic unit (EIE Mini Endo; EMS, Nyon, Switzerland) set at medium power and with copious water irrigation. The root-end cavity preparations were filled with the materials under study. The teeth were filled with the test materials and immersed in HBSS for either 1 day or 28 days. After the immersion period, the tooth to material interface was assessed using a scanning electron microscope. The teeth were embedded in resin, sectioned vertically, polished, and carbon coated. The morphology and composition of the tooth to material interface were assessed under a scanning electron microscope (Hitachi S-4300SE-N; Hitachi Ltd, Tokyo, Japan). Energy-dispersive X-ray spectroscopy (EDS) line analysis was performed across the tooth and the restorative materials, and EDS maps of the region were obtained.
Results Mercury Intrusion Porosimetry The numeric results for MTA-W and MTA-AW at 1 and 28 days are summarized in Table 1. For both material types, porosity decreased with age. MTA-AW had a lower initial (1 day) porosity than MTA-W but a higher porosity after being immersed in HBSS for 28 days. Assessment of Tooth to Material Interface Backscatter micrographs of the polished longitudinal sections of MTA mixed with water after 1 day of immersion in HBSS are shown in Figure 1. Significant porosity was evident in both test materials immersed in HBSS for 1 day, with the voids being on average approximately 40 mm in diameter. The bismuth oxide particles TABLE 1. Mercury Intrusion Porosimetry Test Results MTA-W
MTA-AW
Parameter tested
1 day
28 days
1 day
28 days
Average pore diameter (mm) Porosity %
0.0693
0.0301
0.0325
0.0202
47.18
19.90
37.71
25.50
MTA-AW, MTA Plus mixed with the antiwashout gel; MTA-W, MTA Plus mixed with water.
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Figure 1. Backscatter electron micrographs of polished sections of MTA mixed with water of antiwashout gel after immersion for 1 day or 28 days in HBSS.
appeared evenly distributed and not agglomerated. Less microporosity was evident in the 28-day cured materials compared with the 1-day samples. In the 28-day samples, a gap at the root-dentin interface was evident as opposed to the materials immersed in HBSS for 1 day. EDS maps are shown in Figure 2. The 1-day root-end restorations with both test materials exhibited a high concentration of silicon in the cement, and a high concentration of phosphorus was observed in the tooth. Calcium was observed in both structures. The margin between the tooth and the cement remained sharp in each map. This is an indication that minimal elemental diffusion across the interface occurred. At 28 days, the elemental EDS maps for both materials exhibited a blurred boundary for all 3 elements investigated, showing that diffusion of elements has taken place across the boundary. The tooth exhibited a higher calcium level than the MTA, but the boundary appeared to contain less calcium than either the tooth or the cement. This result may also have been caused by a gap in the boundary that would have interfered with the EDS signal. EDS maps of 28-day MTA-AW exhibited boundaries that were more well defined than for MTA-W; however, a slightly elevated concentration of calcium at the material to tooth boundary was evident. Line scan results for both materials after 1 and 28 days of immersion in HBSS are shown in Figure 3A and B. The line scans indicate a sharp increase in bismuth when coinciding with bismuth oxide particles within MTA. The calcium content was similar for the tooth and cement materials. A dip in the calcium content coincided with the tooth-cement interfacial gap whenever present. The phosphorus line scan showed a region with decreasing phosphorus concentration within the tooth material in contact with MTA. At the tooth-MTA interface, the silicon concentration was lower than the average concentration in MTA but higher than the average of the tooth. This indicates diffusion of silicon from MTA into the dentin. The results for MTA-AW were qualitatively similar to those of MTA-W. The line scan results at 28 days for MTA-W and for MTA-AW indicate a more gradual change in elemental constitution than was observed for the 1-day samples, confirming that elemental diffusion JOE — Volume -, Number -, - 2014
has taken place (even for the bismuth) in both test materials. In both MTA-W and MTA-AW, a silicon-enriched region has formed in the tooth, extending into it for a length of approximately 15 mm, coupled with a silicon-depleted region in the cement of around 15 mm. A phosphorus-depleted, 5-mm thick region was observed in the tooth (with no clear corresponding phosphorus-rich region in the cement), but the transition was sharper for MTA-AW than for MTA-W. Calcium diffusion was slight, and once again a higher calcium level was present in the tooth than in the cement.
Discussion Porosity is a common characteristic of cements and occurs as a result of the spaces between the unhydrated cement grains. As the hydration reaction progresses, the hydration products fill these gaps and reduce the porosity; however, if too high a water to cement ratio is used during mixing, excess water eventually dries off and leaves voids that are not filled by hydration products. Porosity is observed to increase with an increase in water to cement ratio (21) and decreases as the cement ages. Three varieties of pores can be distinguished: closed, through, and blind pores (22). Closed pores are inaccessible to fluids; blind pores terminate inside the material, whereas through pores facilitate the complete passageway of fluids. ‘‘Open porosity’’ includes only through and blind pores. Closed pores have a significant effect on the mechanical properties of the cement, and the open porosity directly impacts the ease with which undesirable oral fluids can penetrate into unprotected dentin (23). Mercury intrusion porosimetry (MIP) has proven to be a valid and reliable technique for evaluating pore size distribution and total open porosity of endodontic materials (24). This technique estimates porosity based on the behavior of a nonwetting liquid (mercury), which does not enter pores spontaneously, but is forced into pores by an external pressure. Because the pressure required for mercury to intrude a pore depends on the diameter of the pore, plotting intrusion volume against intrusion pressure will give information of the pore size distribution and total open porosity of the material.
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Figure 2. EDS maps of MTA-W and MTA-AW root-end restorations stored for 1 day or 28 days in HBSS. Phosphorus can be observed in the MTA and silicon migration in the tooth structure is also evident. Si, silicon; Ca, calcium; P, phosphorous.
The MIP method has certain intrinsic limitations because mercury must pass through the narrowest pores connecting the pore network and therefore cannot provide a true pore size distribution (21). It has to be taken into account that MIP measures the diameter of the pore entrance rather than the pore diameter itself (25), and hence large cavities connected by smaller throats are registered as porosity having the diameter of the throats. The values of total porosity given by MIP can also differ from those obtained by other techniques. The large pores seen in the scanning electron microscopic micrographs were not reflected in the MIP measurements (25). A decrease in pore size was observed between the 1-day and 28-day samples. Cement maturation results in decreased porosity. 4
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The numeric porosity results of MTA-W reported in this study are different than those reported for more established brands of MTA. The reported results for gray MTA-Angelus (26) were 32% porosity at 1 day and 25% at 28 days, with a pore diameter of 2.5 mm. In the current study, 47.1% and 19.9% for 1- and 28-day samples, respectively, and modal pore diameters of 0.43 mm and 0.04 mm for MTA-W at 1 day and 0.05 mm at 28 days are reported. MTA Plus is finer than the other MTAs on the market (15). The findings of the lower initial pore size compared with MTA-Angelus appear to support that statement. The porosity of Bioaggregate (Innovative Bioceramix Inc, Vancouver, BC, Canada) and Biodentine, both tricalcium silicate–based materials, was reported to be 36.9% and 13.4%, JOE — Volume -, Number -, - 2014
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Figure 3. Line scan results for MTA-W and MTA-AW root-end restorations after (A) 1 day and (B) 28 days in HBSS showing the elemental distribution at the interface. The depletion of phosphorus in dentin is evident together with migration of bismuth and silicon from the MTA to the dentin.
respectively, after 28 days of immersion in physiological solution (27). The results of percentage porosity of MTA Plus mixed with both water and antiwashout gel are both lower than Bioaggregate at the same time point but not as low as that reported for Biodentine. Biodentine uses a very low water to powder ratio because of the addition of a water-soluble polymer to the mixing liquid (28). Thus, it is postulated that the antiwashout gel reduces the early material porosity. This feature is particularly important in the clinical context because porosity can contribute to fluid percolation with eventual leakage. In the long-term, with cement aging, the porosity of all MTAs and related materials reduces; thus, there is no clinical concern with the porosity exhibited by MTA Plus mixed with antiwashout gel. JOE — Volume -, Number -, - 2014
MTA-AW was found to have lower porosity than MTA-W at 1 day and higher porosity and a smaller pore size diameter than MTA-W at 28 days. The lower initial porosity of MTA-AW may be caused by differences in the initial hydration rate brought about by the gel, the plasticizing effects by the gel, and the lower actual water to cement ratio of MTA-AW. Further studies are required to elucidate the main reason(s) behind this observation. The scanning electron microscopic/EDS results show a distinct boundary between the MTA and dentin, with significant diffusion of calcium, phosphorus, and silicon between the 2 phases. This diffusion across the interface was consistent with the findings from other studies (7, 29). Calcium and silicon uptake is considered a positive attribute because it is believed to cause chemical and structural modification
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Basic Research—Technology of dentin, which may result in higher acid resistance and physical strength (29). In the current study, particularly in the early ages, a calciumdepleted area coincided with the interfacial area. Furthermore, a concentration of phosphorus in the tooth structure appeared to reduce in areas in contact with the material. The phosphorus found in the material could have originated from the soaking solution. Silicon and bismuth were also shown to migrate in the tooth structure. Thus, element migration was evident with both materials tested. Tricalcium silicate–based materials have been shown to affect the strength of dentin (10, 11). Thus, the depletion of phosphorus from tooth structures needs to be further investigated. The migration of bismuth in tooth structures could be undesirable because MTA and bismuth oxide have been linked with tooth discoloration (30–33). The results of the current study are in accordance with previous research in which the alkaline caustic effect of the calcium silicate cement’s hydration products degraded the collagenous component of the interfacial dentin. This degradation led to the formation of a porous structure, which facilitated the permeation of high concentrations of calcium ions, leading to increased mineralization in this region (34). Besides, the movement of calcium migration of other elements has been shown in this study. The implications of the interaction of MTA with tooth structures need to be further investigated. MTA-AW showed similar results to MTA-W.
Conclusions MTA Plus mixed with antiwashout gel was found to have lower initial porosity than MTA Plus mixed with water. Both materials exhibited good marginal adaptation and diffusion of silicon, calcium, bismuth, and phosphorus across the cement/dentin interface. Thus the anti–washout-type MTA can be considered to be a suitable substitute for ordinary MTA in all its indications.
Acknowledgments The authors thank the Faculty of Dental Surgery and the Research Grant Committee University of Malta for funding; Dr Carolyn Primus for the materials; Mr. Guillaume Potier, Dr. Vincent Thiery, and Mr. Damien Betrancourt of Ecole Des Mines de Douai (France) for their technical expertise. This work was conducted on an internship funded by a grant offered by the Embassy of France to Malta together with the CNRS (French National Centre for Scientific Research) Office of European Research and International Co-Operation and the Malta Council for Science and Technology (MCST) in conjunction with the University of Malta. The authors deny any conflicts of interest related to this study.
References 1. Parirokh M, Torabinejad M. Mineral trioxide aggregate: a comprehensive literature review—part I: chemical, physical, and antibacterial properties. J Endod 2010;36: 16–27. 2. Hilton TJ, Ferracane JL, Mancl L. Northwest Practice-based Research Collaborative in Evidence-based Dentistry (NWP). Comparison of CaOH with MTA for direct pulp capping: a PBRN randomized clinical trial. J Dent Res 2013;92(suppl):16S–22. 3. Damle SG, Bhattal H, Loomba A. Apexification of anterior teeth: a comparative evaluation of mineral trioxide aggregate and calcium hydroxide paste. J Clin Pediatr Dent 2012;36:263–8. 4. Main C, Mirzayan N, Shabahang S, et al. Repair of root perforations using mineral trioxide aggregate: a long-term study. J Endod 2004;30:80–3.
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5. Song M, Kim E. A prospective randomized controlled study of mineral trioxide aggregate and super ethoxy-benzoic acid as root-end filling materials in endodontic microsurgery. J Endod 2012;38:875–9. 6. Torabinejad M, Chivian N. Clinical applications of mineral trioxide aggregate. J Endod 1999;25:197–205. 7. Sarkar NK, Caicedo R, Ritwik P, et al. Physicochemical basis of the biologic properties of mineral trioxide aggregate. J Endod 2005;31:97–100. 8. Reyes-Carmona JF, Felippe MS, Felippe WT. Biomineralization ability and interaction of mineral trioxide aggregate and white portland cement with dentin in a phosphate-containing fluid. J Endod 2009;35:731–6. 9. Tay FR, Pashley DH, Rueggeberg FA, et al. Calcium phosphate phase transformation produced by the interaction of the portland cement component of white mineral trioxide aggregate with a phosphate-containing fluid. J Endod 2007;33:1347–51. 10. Leiendecker AP, Qi YP, Sawyer AN, et al. Effects of calcium silicate-based materials on collagen matrix integrity of mineralized dentin. J Endod 2012;38:829–33. 11. Sawyer AN, Nikonov SY, Pancio AK, et al. Effects of calcium silicate-based materials on the flexural properties of dentin. J Endod 2012;38:680–3. 12. Qi YP, Li N, Niu LN, et al. Remineralization of artificial dentinal caries lesions by biomimetically modified mineral trioxide aggregate. Acta Biomater 2012;8: 836–42. 13. Porter ML, Berto A, Primus CM, et al. Physical and chemical properties of new-generation endodontic material. J Endod 2010;36:524–8. 14. Wang X, Chen L, Xiang H, et al. Influence of anti-washout agents on the rheological properties and injectability of a calcium phosphate cement. J Biomed Mater Res B App Biomater 2007;81:410–8. 15. Formosa LM, Mallia B, Camilleri J. Mineral trioxide aggregate with anti-washout gel properties and microstructure. Dent Mater 2013;29:294–306. 16. Formosa LM, Mallia B, Camilleri J. A quantitative method for determining the anti-washout characteristics of cement-based dental materials including mineral trioxide aggregate. Int Endod J 2013;46:179–86. 17. Formosa LM, Mallia B, Camilleri J. Push-out bond strength of MTA with antiwashout gel or resins. Int Endod J 2013 Jul 12 . http://dx.doi.org/10.1111/iej.12169. 18. Neville AM. Properties of Concrete, 5th ed. Trans-Atlantic Publications, Inc; 2011. 19. Paillere AM. Applications of Admixtures in Concrete. New York: E & FN Spon, An Imprint of Chapman & Hall; 1994. 20. Kadish KM. Proceedings of the Symposium on Recent Advances in the Chemistry and Physics of Fullerenes and Related Materials. Pennington, NJ: The Electrochemical Society; 1994:97. 21. Cook RA, Hover KC. Mercury porosimetry of hardened cement pastes. Cem Concr Res 1999;29:933–43. 22. Webb PA, On C. Analytical Methods in Fine Particle Technology. Norcross: Micrometrics Instrument Corporation; 1997. 23. Milutinovic-Nikolic AD, Medic VB, Vukovic ZM. Porosity of different dental luting cements. Dent Mater 2007;23:674–8. 24. Saghiri MA, Asgar K, Lotfi M, et al. Application of mercury intrusion porosimetry for studying the porosity of mineral trioxide aggregate at two different pH. Acta Odontol Scand 2012;70:78–82. 25. Espanol M, Perez RA, Montufar EB, et al. Intrinsic porosity of calcium phosphate cements and its significance for drug delivery and tissue engineering applications. Acta Biomater 2009;5:2752–62. 26. Oliveira IR, Pandolfelli VC, Jacobovitz M. Chemical, physical and mechanical properties of a novel calcium aluminate endodontic cement. Int Endod J 2010; 43:1069–76. 27. Camilleri J, Grech L, Galea K, et al. Assessment of porosity and sealing ability of tricalcium silicate-based root-end filling materials. Clin Oral Investig 2013 Oct 8. [Epub ahead of print]. 28. Camilleri J, Sorrentino F, Damidot D. Investigation of the hydration and bioactivity of radiopacified tricalcium silicate cement, Biodentine and MTA Angelus. Dent Mater 2013;29:580–93. 29. Han L, Okiji T. Uptake of calcium and silicon released from calcium silicate-based endodontic materials into root canal dentine. Int Endod J 2011;44:1081–7. 30. Bortoluzzi EA, Araujo GS, Guerreiro Tanomaru JM, et al. Marginal gingiva discoloration by gray MTA: a case report. J Endod 2007;33:325–7. 31. Ioannidis K, Mistakidis I, Beltes P, et al. Spectrophotometric analysis of coronal discoloration induced by grey and white MTA. Int Endod J 2013;46:137–44. 32. Felman D, Parashos P. Coronal tooth discoloration and white mineral trioxide aggregate. J Endod 2013;39:484–7. 33. Camilleri J. The color stability of white mineral trioxide aggregate in contact with sodium hypochlorite solution. J Endod (in press). 34. Atmeh AR, Chong EZ, Richard G, et al. Dentin-cement interfacial interaction: calcium silicates and polyalkenoates. J Dent Res 2012;91:454–9.
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Mercury Intrusion Porosimetry and Assessment of Cement-dentin Interface of Anti–washout-type Mineral Trioxide Aggregate L.M. Formosa,* D. Damidot,†‡ and Josette Camilleri, PhD§ Abstract Introduction: One of the disadvantages of mineral trioxide aggregate (MTA) is washout (ie, the tendency of freshly prepared cement paste to disintegrate upon early contact with physiological fluids). A novel MTA (MTA Plus; Prevest Denpro, Jammu City, India) exhibits low washout and superior physical properties when mixed with a gel instead of water. When used as a root-end filler, MTA is in contact with both bone and root dentin. This study aimed to investigate the porosity and interfacial characteristics of the novel MTA mixed with water or antiwashout gel. Methods: Porosity was evaluated after 1 or 28 days of immersion in Hank’s balanced salt solution using mercury intrusion porosimetry. The root dentin to material interface was investigated using a scanning electron microscope and energy-dispersive X-ray spectroscopy complete with line scans and elemental maps. Results: Anti– washout-type MTA Plus was found to have lower initial porosity than MTA Plus mixed with water although this trend was reversed after 28 days of immersion in physiological fluid. Both materials exhibited good marginal adaptation. The diffusion of silicon, calcium, and phosphorus across the cement/dentin interface was observed. Conclusions: MTA Plus mixed with antiwashout gel was found to have lower initial porosity than MTA Plus mixed with water. Both materials exhibited good marginal adaptation and the diffusion of silicon, calcium, and phosphorous across the cement/dentin interface. Thus, the anti–washout-type MTA can be considered to be a suitable substitute for ordinary MTA in all its indications. (J Endod 2014;-:1–6)
Key Words Antiwashout, dentin-material interface, mercury intrusion porosimetry, mineral trioxide aggregate, root-end filling materials
W
hite mineral trioxide aggregate (MTA) is a dental cement with an ability to set in the presence of moisture (1). MTA has numerous applications including pulp capping (2), apexification (3), repair of root perforations (4), root-end filling (5), and others (6). When MTA is used as a root-end filling material, it is in contact with both dentin and bone at the periapex. The interaction of MTA with physiological solutions has been well documented. The Portland cement component of MTA interacts with physiological fluids, and a reaction of calcium hydroxide with phosphates present in tissue fluids occurs (7–9). An amorphous calcium phosphate phase is initially formed and then transformed to an apatite phase, with the latter consisting of calcium-deficient, poorly crystalline, B-type carbonated apatite crystallites. Amorphous calcium phosphate is a key intermediate that precedes biological apatite formation in skeletal calcification (9). The interaction of MTA with dentin also has been documented more recently. The prolonged contact of mineralized dentin with calcium silicate–based cements has an adverse effect on the integrity of the dentin collagen matrix. The amount of collagen extracted was limited to the contact surface (10). Both MTA and Biodentine (Septodont, Saint Maur-des-Fosses, France) caused an alteration of strength and stiffness of dentin (11). Furthermore, calcium silicate–based materials can lead to dentin remineralization in the presence of synthetic tissue fluid (12). One of the drawbacks of MTA when used as a root-end filling material is its poor resistance to washout (13) (ie, the tendency of freshly prepared cement paste to disintegrate upon premature contact with blood or other fluids) (14). Washout of a root-end filling material can occur when rinsing an osteotomy site (13), resulting in a compromised root-end seal. A novel mineral trioxide aggregate (MTA Plus; Prevest Denpro, Jammu City, India) that has a finer particle size than the MTAs currently available for clinical use (15) is supplied with either water or a water-based antiwashout gel intended to be mixed with the MTA powder in place of water to improve the washout resistance. The antiwashout gel is a water-based polymer that when mixed with MTA Plus powder improves the material workability and handling (15). This gel has already been shown to markedly improve the washout resistance of MTA (16). The use of the antiwashout gel together with the finer particle size of the MTA powder results in a material with improved physical and chemical properties. MTA Plus mixed with antiwashout gel exhibited reduced fluid uptake in the early stages of reaction, a reduced setting time, and enhanced compressive strength when compared with the same formulation mixed with water (15). However, the push-out bond strength of MTA Plus mixed with gel was lower than that of the material mixed with water (17). The improvement in the physical properties of Portland cement–based materials is usually related to reductions in the water/cement ratio (18) and the additions of
From the *Department of Metallurgy and Materials Engineering, Faculty of Engineering, University of Malta, Malta; †University of Lille Nord de France, Lille, France; EM Douai, Douai, France; and §Department of Restorative Dentistry, Faculty of Dental Surgery, University of Malta, Malta. Address requests for reprints to Dr Josette Camilleri, Department of Restorative Dentistry, Faculty of Dental Surgery, University of Malta, Medical School, Mater Dei Hospital, Msida MSD 2090, Malta. E-mail address: [email protected] 0099-2399/$ - see front matter Copyright ª 2014 American Association of Endodontists. http://dx.doi.org/10.1016/j.joen.2013.11.015
‡
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Basic Research—Technology polymers that allow a reduction in the water/cement ratio without modifications to the material workability (19). The low water to cement ratio results in a reduction in material porosity. The purpose of this study was to investigate the porosity and interface characteristics of MTA Plus mixed with antiwashout gel (MTA-AW) and to compare these properties with MTA Plus mixed with water, which served as the control (MTA-W).
Materials and Methods The materials used in this study included MTA Plus (compounded by Prevest Denpro, Jammu, India, for Avalon Biomed Inc, Bradenton, FL) lot #2011022801 mixed with either distilled water at a water to powder ratio of 0.35 (MTA-W) or mixed with antiwashout gel (compounded by Prevest Denpro, Jammu, India, for Avalon Biomed Inc) (MTA-AW). The gel was dosed by weight (0.350 g gel per gram MTA Plus powder). The testing was performed on days 1 and 28, and the materials were stored immersed in Hank’s balanced salt solution (HBSS [H6648; Sigma-Aldrich, St Louis, MO]).
Mercury Intrusion Porosimetry Cube specimens (7 7 7 mm) were cast and immersed in HBSS for 1 or 28 days; after this, they were taken out of the solution and dried in a desiccator in the presence of soda lime to avoid surface carbonation. The porosity was measured in a 2-stage process using a mercury intrusion porosimeter (Micromeritics AutoPore IV; Micromeritics France SA, Paris, France). For the first (low pressure) stage, the specimens (of approximate volume 0.343 cm3) were weighed on an analytic balance, transferred into a glass penetrometer cell with a total internal volume of 15.52 cm3 (of which 0.366 cm3 was the stem volume), and the entire assembly was weighed. Precise measurement of the bulk volume of each sample was then performed using a calibrated mercury displacement pycnometer within the porosimeter’s low-pressure chamber. The chamber was evacuated to a pressure of 50 mm Hg over an evacuation time of 300 seconds, and mercury was introduced to the penetrometer at a filling pressure of 0.45 psi. The filled penetrometer was then weighed on an analytic balance and transferred to the high-pressure cell. The pressure was subsequently increased in 60 pressure increments to a maximum pressure of 33,000 psi (227.53 MPa). The pressure increments were chosen to correspond to even intervals on a logarithmic scale. After each pressure increment, the machine was set to pause for 20 seconds to allow the pressure and volume readings to stabilize before recording a pressure/volume data pair. The first stage of testing involved filling the cup and capillary stem with mercury. The pressure on the filled penetrometer was increased, resulting in mercury intrusion into the pores of the test sample and a corresponding decrease of the mercury level in the stem. A capacitance change between the mercury column inside the stem and the metal cladding on the outer surface of the stem allowed the volume of the mercury intruded in the test specimen to be measured to a high degree of accuracy. The pressure was then decreased back down to atmospheric pressure in 25 increments, measuring extrusion volume at each step. Porosimetry data were processed using the software supplied with the machine (Micromeritics France SA, Paris, France), which generates volume and size distributions from the pressure versus intrusion data using a method based on the following Washburn 4 equation: D ¼ 4g cos P , where D is the pore diameter, g is the surface tension of mercury (485 dynes/cm), P is the applied pressure, and F is the contact angle (130 ) between the mercury and the sample. 2
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The average pore diameter (4V/A) was given directly by the porosimeter. It was assumed that all pores are cylindrical in shape. Hence, by dividing the pore volume (V = pd2L/4) by the pore area (A = pdL), the average pore diameter (d) was determined (equal to 4V/A) (20). The bulk and skeletal densities were calculated. Bulk density is the density of the cement specimens as a whole, which includes the volume occupied by the pores. Skeletal density represents the density of the cement ‘‘skeleton’’ without the pores. The total percentage porosity was calculated from the difference between the bulk and skeletal densities.
Assessment of Tooth to Material Interface Single-rooted teeth extracted for periodontal reasons were used. The teeth were decoronated, and the root length was standardized to 15 mm. The root canals were instrumented with ProTaper rotary nickel-titanium instruments (Dentsply Maillefer, Ballaigues, Switzerland) and irrigated with 10 mL 5% sodium hypochlorite followed by 2 mL EDTA to remove the smear layer. The canals were dried and obturated with gutta-percha and AH Plus sealer (Dentsply Maillefer) using the warm vertical condensation technique (System B; SybronEndo, Orange, CA). The coronal access was restored with glass ionomer cement (Fuji IX; GC Europe, Leuven, Belgium). The root ends were resected and root-end preparations were prepared to a depth of 3 mm using an ultrasonic tip (KiS-ID; Obtura-Spartan, Earth City, MO) attached to an ultrasonic unit (EIE Mini Endo; EMS, Nyon, Switzerland) set at medium power and with copious water irrigation. The root-end cavity preparations were filled with the materials under study. The teeth were filled with the test materials and immersed in HBSS for either 1 day or 28 days. After the immersion period, the tooth to material interface was assessed using a scanning electron microscope. The teeth were embedded in resin, sectioned vertically, polished, and carbon coated. The morphology and composition of the tooth to material interface were assessed under a scanning electron microscope (Hitachi S-4300SE-N; Hitachi Ltd, Tokyo, Japan). Energy-dispersive X-ray spectroscopy (EDS) line analysis was performed across the tooth and the restorative materials, and EDS maps of the region were obtained.
Results Mercury Intrusion Porosimetry The numeric results for MTA-W and MTA-AW at 1 and 28 days are summarized in Table 1. For both material types, porosity decreased with age. MTA-AW had a lower initial (1 day) porosity than MTA-W but a higher porosity after being immersed in HBSS for 28 days. Assessment of Tooth to Material Interface Backscatter micrographs of the polished longitudinal sections of MTA mixed with water after 1 day of immersion in HBSS are shown in Figure 1. Significant porosity was evident in both test materials immersed in HBSS for 1 day, with the voids being on average approximately 40 mm in diameter. The bismuth oxide particles TABLE 1. Mercury Intrusion Porosimetry Test Results MTA-W
MTA-AW
Parameter tested
1 day
28 days
1 day
28 days
Average pore diameter (mm) Porosity %
0.0693
0.0301
0.0325
0.0202
47.18
19.90
37.71
25.50
MTA-AW, MTA Plus mixed with the antiwashout gel; MTA-W, MTA Plus mixed with water.
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Figure 1. Backscatter electron micrographs of polished sections of MTA mixed with water of antiwashout gel after immersion for 1 day or 28 days in HBSS.
appeared evenly distributed and not agglomerated. Less microporosity was evident in the 28-day cured materials compared with the 1-day samples. In the 28-day samples, a gap at the root-dentin interface was evident as opposed to the materials immersed in HBSS for 1 day. EDS maps are shown in Figure 2. The 1-day root-end restorations with both test materials exhibited a high concentration of silicon in the cement, and a high concentration of phosphorus was observed in the tooth. Calcium was observed in both structures. The margin between the tooth and the cement remained sharp in each map. This is an indication that minimal elemental diffusion across the interface occurred. At 28 days, the elemental EDS maps for both materials exhibited a blurred boundary for all 3 elements investigated, showing that diffusion of elements has taken place across the boundary. The tooth exhibited a higher calcium level than the MTA, but the boundary appeared to contain less calcium than either the tooth or the cement. This result may also have been caused by a gap in the boundary that would have interfered with the EDS signal. EDS maps of 28-day MTA-AW exhibited boundaries that were more well defined than for MTA-W; however, a slightly elevated concentration of calcium at the material to tooth boundary was evident. Line scan results for both materials after 1 and 28 days of immersion in HBSS are shown in Figure 3A and B. The line scans indicate a sharp increase in bismuth when coinciding with bismuth oxide particles within MTA. The calcium content was similar for the tooth and cement materials. A dip in the calcium content coincided with the tooth-cement interfacial gap whenever present. The phosphorus line scan showed a region with decreasing phosphorus concentration within the tooth material in contact with MTA. At the tooth-MTA interface, the silicon concentration was lower than the average concentration in MTA but higher than the average of the tooth. This indicates diffusion of silicon from MTA into the dentin. The results for MTA-AW were qualitatively similar to those of MTA-W. The line scan results at 28 days for MTA-W and for MTA-AW indicate a more gradual change in elemental constitution than was observed for the 1-day samples, confirming that elemental diffusion JOE — Volume -, Number -, - 2014
has taken place (even for the bismuth) in both test materials. In both MTA-W and MTA-AW, a silicon-enriched region has formed in the tooth, extending into it for a length of approximately 15 mm, coupled with a silicon-depleted region in the cement of around 15 mm. A phosphorus-depleted, 5-mm thick region was observed in the tooth (with no clear corresponding phosphorus-rich region in the cement), but the transition was sharper for MTA-AW than for MTA-W. Calcium diffusion was slight, and once again a higher calcium level was present in the tooth than in the cement.
Discussion Porosity is a common characteristic of cements and occurs as a result of the spaces between the unhydrated cement grains. As the hydration reaction progresses, the hydration products fill these gaps and reduce the porosity; however, if too high a water to cement ratio is used during mixing, excess water eventually dries off and leaves voids that are not filled by hydration products. Porosity is observed to increase with an increase in water to cement ratio (21) and decreases as the cement ages. Three varieties of pores can be distinguished: closed, through, and blind pores (22). Closed pores are inaccessible to fluids; blind pores terminate inside the material, whereas through pores facilitate the complete passageway of fluids. ‘‘Open porosity’’ includes only through and blind pores. Closed pores have a significant effect on the mechanical properties of the cement, and the open porosity directly impacts the ease with which undesirable oral fluids can penetrate into unprotected dentin (23). Mercury intrusion porosimetry (MIP) has proven to be a valid and reliable technique for evaluating pore size distribution and total open porosity of endodontic materials (24). This technique estimates porosity based on the behavior of a nonwetting liquid (mercury), which does not enter pores spontaneously, but is forced into pores by an external pressure. Because the pressure required for mercury to intrude a pore depends on the diameter of the pore, plotting intrusion volume against intrusion pressure will give information of the pore size distribution and total open porosity of the material.
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Figure 2. EDS maps of MTA-W and MTA-AW root-end restorations stored for 1 day or 28 days in HBSS. Phosphorus can be observed in the MTA and silicon migration in the tooth structure is also evident. Si, silicon; Ca, calcium; P, phosphorous.
The MIP method has certain intrinsic limitations because mercury must pass through the narrowest pores connecting the pore network and therefore cannot provide a true pore size distribution (21). It has to be taken into account that MIP measures the diameter of the pore entrance rather than the pore diameter itself (25), and hence large cavities connected by smaller throats are registered as porosity having the diameter of the throats. The values of total porosity given by MIP can also differ from those obtained by other techniques. The large pores seen in the scanning electron microscopic micrographs were not reflected in the MIP measurements (25). A decrease in pore size was observed between the 1-day and 28-day samples. Cement maturation results in decreased porosity. 4
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The numeric porosity results of MTA-W reported in this study are different than those reported for more established brands of MTA. The reported results for gray MTA-Angelus (26) were 32% porosity at 1 day and 25% at 28 days, with a pore diameter of 2.5 mm. In the current study, 47.1% and 19.9% for 1- and 28-day samples, respectively, and modal pore diameters of 0.43 mm and 0.04 mm for MTA-W at 1 day and 0.05 mm at 28 days are reported. MTA Plus is finer than the other MTAs on the market (15). The findings of the lower initial pore size compared with MTA-Angelus appear to support that statement. The porosity of Bioaggregate (Innovative Bioceramix Inc, Vancouver, BC, Canada) and Biodentine, both tricalcium silicate–based materials, was reported to be 36.9% and 13.4%, JOE — Volume -, Number -, - 2014
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Figure 3. Line scan results for MTA-W and MTA-AW root-end restorations after (A) 1 day and (B) 28 days in HBSS showing the elemental distribution at the interface. The depletion of phosphorus in dentin is evident together with migration of bismuth and silicon from the MTA to the dentin.
respectively, after 28 days of immersion in physiological solution (27). The results of percentage porosity of MTA Plus mixed with both water and antiwashout gel are both lower than Bioaggregate at the same time point but not as low as that reported for Biodentine. Biodentine uses a very low water to powder ratio because of the addition of a water-soluble polymer to the mixing liquid (28). Thus, it is postulated that the antiwashout gel reduces the early material porosity. This feature is particularly important in the clinical context because porosity can contribute to fluid percolation with eventual leakage. In the long-term, with cement aging, the porosity of all MTAs and related materials reduces; thus, there is no clinical concern with the porosity exhibited by MTA Plus mixed with antiwashout gel. JOE — Volume -, Number -, - 2014
MTA-AW was found to have lower porosity than MTA-W at 1 day and higher porosity and a smaller pore size diameter than MTA-W at 28 days. The lower initial porosity of MTA-AW may be caused by differences in the initial hydration rate brought about by the gel, the plasticizing effects by the gel, and the lower actual water to cement ratio of MTA-AW. Further studies are required to elucidate the main reason(s) behind this observation. The scanning electron microscopic/EDS results show a distinct boundary between the MTA and dentin, with significant diffusion of calcium, phosphorus, and silicon between the 2 phases. This diffusion across the interface was consistent with the findings from other studies (7, 29). Calcium and silicon uptake is considered a positive attribute because it is believed to cause chemical and structural modification
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Basic Research—Technology of dentin, which may result in higher acid resistance and physical strength (29). In the current study, particularly in the early ages, a calciumdepleted area coincided with the interfacial area. Furthermore, a concentration of phosphorus in the tooth structure appeared to reduce in areas in contact with the material. The phosphorus found in the material could have originated from the soaking solution. Silicon and bismuth were also shown to migrate in the tooth structure. Thus, element migration was evident with both materials tested. Tricalcium silicate–based materials have been shown to affect the strength of dentin (10, 11). Thus, the depletion of phosphorus from tooth structures needs to be further investigated. The migration of bismuth in tooth structures could be undesirable because MTA and bismuth oxide have been linked with tooth discoloration (30–33). The results of the current study are in accordance with previous research in which the alkaline caustic effect of the calcium silicate cement’s hydration products degraded the collagenous component of the interfacial dentin. This degradation led to the formation of a porous structure, which facilitated the permeation of high concentrations of calcium ions, leading to increased mineralization in this region (34). Besides, the movement of calcium migration of other elements has been shown in this study. The implications of the interaction of MTA with tooth structures need to be further investigated. MTA-AW showed similar results to MTA-W.
Conclusions MTA Plus mixed with antiwashout gel was found to have lower initial porosity than MTA Plus mixed with water. Both materials exhibited good marginal adaptation and diffusion of silicon, calcium, bismuth, and phosphorus across the cement/dentin interface. Thus the anti–washout-type MTA can be considered to be a suitable substitute for ordinary MTA in all its indications.
Acknowledgments The authors thank the Faculty of Dental Surgery and the Research Grant Committee University of Malta for funding; Dr Carolyn Primus for the materials; Mr. Guillaume Potier, Dr. Vincent Thiery, and Mr. Damien Betrancourt of Ecole Des Mines de Douai (France) for their technical expertise. This work was conducted on an internship funded by a grant offered by the Embassy of France to Malta together with the CNRS (French National Centre for Scientific Research) Office of European Research and International Co-Operation and the Malta Council for Science and Technology (MCST) in conjunction with the University of Malta. The authors deny any conflicts of interest related to this study.
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