Basic Research—Technology
The Effect of Setting Accelerator on Properties of Mineral Trioxide Aggregate Tsui-Hsien Huang, DDS, PhD,* Ming-You Shie, MS,* Chia-Tze Kao, DDS, PhD,† and Shinn-Jyh Ding, PhD† Abstract The purpose of this study was to investigate the effect of a sodium phosphate dibasic (Na2HPO4) setting accelerator on the properties of white-colored mineral trioxide aggregate (WMTA). Setting times were measured by using a Gilmore needle. Changes in the pH value, diametral tensile strength, and phase composition of WMTA were evaluated. By using a 15% Na2HPO4 solution as a liquid phase mixed with WMTA, the setting time was significantly reduced to 26 minutes rather than the usual 3 hours. The 15% Na2HPO4 promoted WMTA to achieve a maximum diametral tensile strength of 4.9 MPa at an initial 6-hour aging time and 1 MPa for the cement mixed with water. The pH value of the 15% Na2HPO4-mixed cement was changed from an initial 11.0 to a high of 13.2, which was similar to the results using water. The results suggest that the Na2HPO4 solution may be an effective setting accelerator for WMTA. (J Endod 2008;34:590–593)
Key Words Mineral trioxide aggregate, root-end filling material, setting time
From the *Institute of Stomatology and the †Institute of Oral Materials Science, Chung-Shan Medical University, Taichung, Taiwan, Republic of China. Address requests for reprints to Dr Shinn-Jyh Ding, Institute of Oral Materials Science, Chung-Shan Medical University, Taichung 402, Taiwan, Republic of China. E-mail address: sjding@csmu.edu.tw 0099-2399/$0 - see front matter Copyright © 2008 by the American Association of Endodontists. doi:10.1016/j.joen.2008.02.002
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variety of biomaterials have been suggested as potential root-end filling materials including silver amalgam, gutta-percha, glass ionomer cements, zinc oxide eugenol-based cements, and mineral trioxide aggregate (MTA) (1–5). Among these filler materials, MTA was more biocompatible than the other root-end filling materials and was suitable for use in clinical applications (5–7). MTA has been proved to stimulate cytokine release from bone cells that actively promote hard-tissue formation (8). In addition, it was highly bioactive, thus inducing strong bonding with bone tissue (9). Although MTA has met most of the requirements of an ideal root-end filling material, its working properties were less than ideal (10 –12). The MTA powder consisted of small, hydrophilic particles, which can set in the presence of water. The setting time of MTA has been reported to be more than 2 hours (2, 13). To improve the working properties of MTA, Kogan et al (12) added various additives such as chlorhexidine, K-Y Jelly (Johnson & Johnson, New Brunswick, NJ), NaOCl gel, and CaCl2 to the gray-colored ProRoot MTA powder (Dentsply Tulsa Dental, Tulsa, OK). The reagents used decreased the setting time after mixing with water, which dropped from the original 50 minutes down to 20 minutes. However, there was also a reduction in the compressive strength from 31% to 40%. Another approach to reduce the setting time was to use a setting accelerator as the liquid phase because a liquid was necessary for hardening powder. Wiltbank et al (14) found that calcium chloride and calcium formate significantly accelerated white-colored MTA (WMTA) in terms of the initial setting time. In this study, the aim of reducing the setting time of WMTA without adverse diametral tensile strength (DTS) was pursued.
Materials and Methods Setting Time and pH Variation The ProRoot WMTA was obtained from Dentsply Tulsa Dental (Tulsa, OK). As per the manufacturer’s instructions, a liquid/powder ratio of 0.3 mL/g was used. In addition to water (the control) as a liquid phase, 0.9% sodium chloride (NaCl), 1 mol/L tris(hydroxymethyl)aminomethane-hydrochloride (Tris-HCl) buffer, 15% sodium hydroxide (NaOH), and 15% calcium hydroxide (Ca[OH]2) were used to harden the WMTA powder (Table 1). Specifically, the liquid accelerator was an aqueous solution of 5%, 10%, and 15% Na2HPO4 (JT Baker, Phillipsburg, NJ). In addition, 15% sodium phosphate monobasic (NaH2PO4) was used as the liquid phase. After mixing the WMTA, cement was placed into a cylindrical stainless steel mold (diameter 6 mm and height 5 mm) and stored in an incubator at 100% relative humidity and 37oC for measurements of the final setting time, pH value, and DTS. The final setting time of specimens was tested at various intervals by using a Gilmore needle of 453 g. The setting time was recorded when the needle failed to create an indentation of 1 mm in depth in three separate areas. The variation in the pH value of the WMTA cement during the setting process was measured with a pH meter (IQ120 miniLab pH meter; IQ Scientific Instruments, San Diego, CA). Three parallel experiments were performed with the data of every group. Diametral Tensile Strength After mixing, WMTA was allowed to set for different predetermined periods of time to examine the set reaction of WMTA through diametral tensile testing. The testing was conducted on an EZ-Test machine (Shimadzu, Kyoto, Japan) at a loading rate of 0.5 mm/min. The DTS value of the specimens was calculated from the relationship DTS
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Basic Research—Technology TABLE 1. Final Setting Times of WMTA after Mixing with Various Liquids Liquid Water NaCl Tris-HCl buffer Na2HPO4 NaH2PO4 NaOH Ca(OH)2
Concentration
Setting time (min) a
0.9% 1 mol/L, pH 7.4 5%,pH 8.8 10%,pH 9.1 15%,pH 9.5 15%,pH 4.4 15%,pH12.9 15%,pH12.4
151 " 6 154 " 7a 155 " 6a 108 " 5b 89 " 4c 26 " 2d 33 " 3d 148 " 3a 150 " 2a
WMTA, white-colored mineral trioxide aggregate. Values are mean " standard deviation. At least three samples were measured for each data point. Mean values followed by the same superscript letter were not significantly different (p # 0.05) according to Scheffe’s post hoc multiple comparisons.
2P/ bw, where P is the peak load (N), b is the diameter (mm), and w is the thickness (mm) of the specimen. Three specimens were tested for each group.
Phase To further investigate the relationship between phase composition and aging time for setting, after the predetermined time periods, the specimens were immediately placed in a tube containing absolute ethanol, cooled to 80oC, and freeze-dried, as per Fukase’s technique (15). Thus, the WMTA reaction with liquid would be stopped. The freeze-dried specimens were ground to fine powders for characterization using an X-ray diffractometer (XRD; Rigaku D/MAX2500, Tokyo, Japan). Statistical Analysis One-way ANOVA statistical analysis was used to evaluate the statistical significance of the measurement data. Scheffe’s multiple comparison testing was used to determine the significance of the deviations in the measurement data of each group. In all cases, the results were considered statistically different at p ! 0.05.
Results Setting Time The results of the setting time for WMTA after mixing with various liquids are summarized in Table 1.When mixed with water, the setting time of set WMTA was as high as 151 minutes. All nonphosphate solutions, such as 0.9% NaCl, Tris-HCl buffer, 15% NaOH, and 15% Ca(OH)2 had the setting time of about 150 minutes. In contrast, the phosphate solutions significantly reduced the setting time down to 108 (p 0.000), 89 (p 0.000), 26 (p 0.000), and 33 minutes (p 0.000) for 5%, 10%, 15%, and 15% NaH2PO4, respectively. Because the Na2HPO4 solution is an effective setting accelerator, the following studies focused on the effect of Na2HPO4 solution on the properties of WMTA.
DTS The effect of Na2HPO4 concentration on the mechanical strength was assessed to determine the time necessary to reach the maximum strength. The DTS values of WMTA as a function of aging time are listed in Table 2. When mixed with water, the DTS at 3-, 6-, 12-, and 24-hour aging was 0.2 " 0, 1.0 " 0.1, 3.5 " 0.2, and 4.4 " 0.1 MPa, respectively. These values are significantly different from each other (p ! 0.05). In the case of 15% Na2HPO4, after just 30 minutes, WMTA could achieve 0.7 MPa, higher than that obtained for the 3-hour aged specimens at the other three conditions. The maximum DTS of 4.9 MPa was obtained at 6-hour aging; its strength tended to the steady value.
Phase Composition The XRD system has been widely used in a nondestructive analysis to identify the phase composition of materials. XRD patterns of WMTA obtained at various time intervals during setting are shown in Figure 1. It can be seen that the 24-hour specimens mixed with water have a higher diffraction peak at 2! 29.3o compared with the other aged specimens (Fig. 1A). However, as the aging time increased, the intensity of the peak at 2! 29.3o for Na2HPO4-mixed cement specimens increased (Fig. 1B). pH Variation There was no significant difference (p # 0.05) in the pH value of WMTA mixed with water or 15% Na2HPO4 solution at the same time periods. The pH value of the two cements at fresh mixing was 11.0. At the 20-minute setting, it reached a value of 12.0, before approaching a steady state up to a pH value of 13.2.
Discussion Over the last decade, much effort has been devoted to investigating the properties of WMTA. Although it seemed to be a good root-end filling material, the long setting time was a shortcoming in the clinical applications (5, 12). In this study, a setting time of 151 minutes after mixing with water was comparable with the values obtained in the previous studies (2, 13). Contrary to the findings on the water factor, it can easily be seen that the phosphate solution brought about the setting acceleration of the WMTA cement. In particular, the final setting time of the cement using either 15% Na2HPO4 solution (pH 9.5) or NaH2PO4 (pH 4.4) could be reduced by a factor of five as compared with that with water. Setting experiments on other liquid phases such as NaOH (pH 12.9), Ca(OH)2 (pH 12.4), 0.9% NaCl (pH 5.4), and Tris-HCl buffer (pH 7.4) showed the non-pH effect on setting time. This further indicated that the observed setting reactivity of WMTA depended on the phosphate effect. Although there was no significant difference in the setting time between 15% Na2HPO4 (pH 9.5) and 15% NaH2PO4 (pH 4.4) (Table 1), the former might be a better choice as a setting accelerator in terms of pH value.
TABLE 2. DTSs of WMTA Mixed with Water or with the Three Different Concentrations of Na2HPO4 Solution versus Aging Time during Setting DTSs (MPa)
Liquid Water Na2HPO4
5% 10% 15%
0.5 hour
1 hour
3 hours
6 hours
12 hours
24 hours
— — — 0.7 " 0.1a,b
— — — 1.8 " 0.4f
0.2 " 0a 0.6 " 0a,b 0.6 " 0.1a,b 3.3 " 0.2c
1.0 " 0.1b 3.0 " 0.2c 4.7 " 0.2d,e 4.9 " 0.1d,e
3.5 " 0.2c 4.3 " 0.2d 4.7 " 0.1d,e 4.9 " 0.1e
4.4 " 0.1d,e 4.7 " 0.1d,e 4.9 " 0.1e 4.8 " 0.2d,e
DTS, diametral tensile strength; WMTA, white-colored mineral trioxide aggregate. Values are mean " standard deviation. Three samples were measured for each data point. Mean values followed by the same superscript letter were not significantly different (p # 0.05) according to Scheffe’s post hoc multiple comparisons.
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Basic Research—Technology phases. Many studies have proved that Na2HPO4 solution could be used as a cement liquid to accelerate the setting of the cement prepared from "-tricalcium phosphate as a cement powder (18, 19). Chow et al (20) have suggested that the rate of hydroxyapatite formation, an indicator of the setting reaction in the calcium phosphate cement system, could be increased by the presence of phosphate in the solution. Li and White (21) found that phosphate solution enabled the network to crosslink in the glass-ionomer matrix. In short, the phosphate solution may influence the kinetics of the setting reaction, as indicated by the increased intensity of the diffraction peak at 2! 29.3o and, subsequently, the amount and properties of particular hydration products. The changes in DTS as a function of aging time (Table 2) follow a pattern that is essentially similar to those observed in the intensity of the diffraction peak at 2! 29.3o. The correlation between the two suggested that the formation of calcium silicate hydrates and/or phosphatecontaining phases might possibly be responsible for the mechanical strength of WMTA. When mixing with water, the strength of 3-hour specimens was 0.2 MPa, and the strength was 4.4 MPa after 24-hour aging. It seemed that most of the activated fraction within WMTA remained unreacted during the initial setting reaction of 3 hours, resulting in a very weak entanglement within WMTA particles. Complete hydration could be achieved in 6 hours for 15% Na2HPO4-mixed specimens compared with 23% for water-mixed WMTA at the same period in terms of the strength. Concerning the variations in pH value during setting, this initial increase in pH value, consistent with another report (13), was a result of the formation of Ca(OH)2 during the setting reaction, which was released to an aqueous environment (16, 22). The high pH value was not considered problematic because it was desirable to destroy any bacterial colonies that might have been present (23). In conclusion, the 15% Na2HPO4 solution was recommend as a liquid phase for WMTA because at that concentration the cement’s final setting time (26 minutes) was suitable for clinical use and it had a good DTS value. Further in vitro studies are currently ongoing to validate the clinical applications.
References
Figure 1. XRD patterns of WMTA mixed with water (A) or 15% Na2HPO4 solution (B) versus aging time during setting. It is worthwhile to note the changes in the diffraction peak at 2! 29.4o, particularly for the Na2HPO4 group.
The WMTA powder mainly consisted of bismuth oxide, tricalcium silicate, dicalcium silicate, and tricalcium aluminate (11). Lea (16) has suggested that tricalcium silicate and dicalcium silicate reacted with water to produce poorly crystallized calcium silicate hydrates and calcium hydroxide. In this study, although the phase evolution of WMTA hydration with Na2HPO4 was similar to that with water during setting (Fig. 1), the intensity of the diffraction peak at 2! 29.3o seemed to be higher than the values obtained in a water system at the same aging time. The diffraction peak at 2! 29.3o was ascribed to unreacted tricalcium silicate (11), calcium silicate hydrates that were formed (17), and/or new phosphate-containing phases that increasingly overlapped each other with increasing aging time. Although it is not clear why this occurs, this role of phosphate could be explained by the strong ionic interactions between phosphate and calcium and/or silicate cations, thereby enhancing the formation of calcium silicate hydrates or other 592
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Basic Research—Technology 14. Wiltbank KB, Schwartz SA, Schindler WG. Effect of selected accelerants on the physical properties of mineral trioxide aggregate and Portland cement. J Endod 2007;33:1235– 8. 15. Fukase Y, Eanes ED, Takagi S, Chow LC, Brown WE. Setting reactions and compressive strengths of calcium phosphate cements. J Dent Res 1990;69:1852– 6. 16. Lea FM. The chemistry of cement and concrete, 3rd ed. London: Edward Arnold; 1970;177– 86. 17. Taylor HFW. Hydrated calcium silicates: part I. Compound formation at ordinary temperatures. J Chem Soc 1950;25:3682–90. 18. Fernández E, Boltong MG, Ginebra MP, Bermúdez O, Driessens FCM, Planell JA. Common ion effect on some calcium phosphate cements. Clin Mater 1994;16:99 –103.
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19. Kon M, Miyamoto Y, Asaoka K, Ishikawa K, Lee HY. Development of calcium phosphate cement for rapid crystallization to apatite. Dent Mater J 1998;17:223–32. 20. Chow LC, Takagi S, Ishikawa K. Formation of hydroxyapatite in cement systems. In: Brown PW, ed. Hydroxyapatite and related materials. Boca Raton, FL: CRC Press1994:127–37. 21. Li ZC, White SN. Mechanical properties of dental luting cements. J Prosthet Dent 1999;81:597– 609. 22. Camilleri J, Montesin FE, Brady K, Sweeney R, Curtis RV, Pitt Ford TR. The constitution of mineral trioxide aggregate. Dent Mater 2005;21:297–303. 23. Brown PW. Hydration behavior of calcium phosphates is analogous to hydration behavior of calcium silicates. Cement Concrete Res 1999;29:1167–71.
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