Physicochemical properties of calcium silicate cements for endodontic treatment

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Basic Research—Technology

Physicochemical Properties of Calcium Silicate Cements for Endodontic Treatment Chun-Cheng Chen, DDS, PhD,* Chia-Che Ho, MS,† Chan-Hen David Chen, PhD,‡ and Shinn-Jyh Ding, PhD† Abstract Introduction: The purpose of this study was to examine the physicochemical properties of novel calcium silicate cements (CSCs) prepared by using a sol-gel method. Methods: The compressive strength, morphology, and phase composition of various cements were evaluated after mixing with water, in addition to setting time and pH value. Results: As solid phases, the sol-gel– derived powders mainly consisted of b-dicalcium silicate. Setting times for cements mixed with water ranged from 12–42 minutes and were lower for cements with higher starting CaO content. The compressive strength of the CSCs ranged from 0.3–15.2 MPa; these values were significantly different (P < .05). Calcium silicate hydrate (C S H) was the principal phase that formed in the hydration process. The CSCs’ pH values changed from an initial 11 to a high of 13. Conclusions: CSCs display advantageously shortened setting times and might have potential for endodontic use, although further tests are necessary to confirm this. (J Endod 2009;35:1288–1291)

Key Words Calcium silicate, mineral trioxide aggregate, Portland cement, root-canal filling material

From the *Department of Dentistry and †Institute of Oral Biology and Biomaterials Science, Chung-Shan Medical University; and ‡Institute of Veterinary Microbiology, National Chung-Hsing University, Taichung, Taiwan, Republic of China. Address requests for reprints to Prof Shinn-Jyh Ding, Institute of Oral Biology and Biomaterials Science, Chung-Shan Medical University, Taichung 402, Taiwan, Republic of China. E-mail address: [email protected] 0099-2399/$0 - see front matter Crown Copyright ª 2009 Published by Elsevier Inc. on behalf of the American Association of Endodontists. doi:10.1016/j.joen.2009.05.036

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alcium silicate–based Portland cement, which has been shown to be biocompatible (1–4), contains tricalcium silicate, dicalcium silicate, and tricalcium aluminate. Mineral trioxide aggregate (MTA) powder is basically a mixture of Portland cement and bismuth (III) oxide (5, 6) and has been used successfully in dental applications for the past decade. MTA is composed of a variety of oxide components, typically SiO2, CaO, and Al2O3. In a previous study (7), SiO2, CaO, and Al2O3 were used to construct a new MTA-like material by high-temperature solid state sintering. The MTA-like cement displayed an advantageously shortened setting time, although it was weaker than white-colored MTA. Among the oxides, aluminum (Al), a neurotoxin, is detrimental to human health because of its ability to disrupt cellular calcium homeostasis (8) and promote cellular oxidation (9). It has been suggested that Al might contribute to Parkinson’s and Alzheimer’s disease (10). Therefore, removal of Al from the cement is necessary. The sol-gels are transformed into ceramics by heating at relatively low temperatures and have better chemical and structural homogeneity than ceramics obtained by conventional glass melting or ceramic powder methods such as solid state sintering. It has been reported that materials prepared by a sol-gel process are more bioactive than materials of the same compositions prepared by other methods (11). The purpose of this study was to examine the physicochemical properties of Al-free calcium silicate cements (CSCs), which powders were prepared by using a sol-gel method, with different molar ratios of SiO2:CaO ranging from 7:3–3:7.

Materials and Methods Specimen Preparation The sol-gel method has been described elsewhere (12). Reagent grade tetraethyl orthosilicate (Si(OC2H5)4; Sigma-Aldrich, St Louis, MO) and calcium nitrate (Ca(NO3)2$4H2O; Showa, Tokyo, Japan) were used as precursors for SiO2 and CaO, respectively. Nitric acid was used as the catalyst and ethanol as the solvent. For simplicity, throughout this study, the sintered powders and the cements derived from such powders were designated by the same codes, as listed in Table 1. Briefly, Si(OC2H5)4 was hydrolyzed with the sequential addition of 2 mol/L HNO3 and absolute ethanol, with 1 hour of stirring separately. The required amount of Ca(NO3)2$4H2O was added to the above solution, and the mixed solutions were stirred for an additional hour. The sol solution was sealed and aged at 60  C for 1 day. After vaporization of the solvent in an oven at 120  C, the dried gel was heated in air to 800  C at a heating rate of 10  C/min for 2 hours by using a high-temperature furnace and then cooled to room temperature in the furnace to produce a powder. The sintered granules were then ball-milled for 12 hours in ethyl alcohol by using a Retsch S 100 centrifugal ball mill (Hann, Germany) and dried in an oven at 60  C. Phase Composition and Microstructure To investigate the phase composition, the specimens were ground to fine powders and then characterized with an x-ray diffractometer (XRD; Shimadzu XD-D1, Kyoto, Japan). Fourier transform infrared spectroscopy (FTIR, Bomem DA8.3; Hartman & Braun, Quebec, Canada) was used to analyze the powders. Scanning electron microscopy (SEM; JEOL JSM-6700F, Tokyo, Japan) was used to characterize the microstructure of the various specimens.

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Basic Research—Technology TABLE 1. Composition (molar ratio), Setting Time (Ts), and Compressive Strength (CS) of the 5 CSCs after Mixing with Water Specimen code

Composition (SiO2:CaO)

Ts (min)

CS (MPa)

S70C30 S60C40 S50C50 S40C60 S30C70

7:3 6:4 5:5 4:6 3:7

42  2a 31  2b 24  2c 16  2d 12  2e

0.3  0.1f 9.4  2.1g 15.2  2.5h 12.0  2.6i 3.2  0.5j

Values are mean  standard deviation. Eight specimens were measured for setting time data. At least 20 specimens were used for CS measurement. Mean values followed by the same superscript letter were not significantly different (P > .05) according to Scheffe´ post hoc multiple comparisons.

Setting Time and pH Variation All specimens were hand-mixed at a liquid-to-powder ratio of 0.5 g/mL, with distilled water as the liquid phase. After mixing, the cement was placed into a cylindrical stainless steel mold (diameter, 6 mm and height, 12 mm) and stored in an incubator at 100% relative humidity and 37 C. The setting times of the cements were tested by using a 400-gauge Gillmore needle with a 1-mm diameter, according to ISO 9917-1 for water-based cements. The pH values of the cement specimens during the setting process were measured with a pH meter (IQ120 miniLab pH meter; IQ Scientific Instruments, San Diego, CA). Triplicate measurements were used. Compressive Strength Compressive strength (CS) was measured on an EZ-Test machine (Shimadzu) at a loading rate of 0.5 mm/minute. Statistical Analysis One-way analysis of variance statistical analysis was used to evaluate the significance of differences between the mean CS or setting time values. Scheffe´ multiple comparison testing was used to determine the significance of the deviations in the data for each specimen. In all cases, the results were considered statistically significant at a P value less than .05.

Results Phase Composition and Morphology Fig. 1A shows the XRD patterns of the 5 SiO2–CaO powders sintered at 800  C and indicates that the phase evolution is dependent on the Si/Ca ratio of the precursors. The major diffraction peaks at 2q between 32 and 34 were attributed to the b-dicalcium silicate (b-Ca2SiO4) phase. The peak intensities of b-Ca2SiO4 and CaO increased with increasing CaO content in the precursors. The trends in the FTIR spectra (Fig. 1B) are similar to those indicated by XRD. For the specimen with the greatest amount of silica (S70C30), the broad IR absorption band corresponding to SiO4 asymmetric stretching extended over a wide wave-number range of 1300–950 cm–1. An obvious sharpening and shifting to lower frequency in Si–O–Si asymmetric stretching bands were detected as the silica content decreased. When the powder solid was mixed with water, the products of the hydration process were C–S–H at 29.3 and incompletely reacted inorganic component phases (Fig. 2A). The lower the Si/Ca ratio was in the precursor, the higher the C–S–H content was in the cement. Except for S70S30 cement, which had a loose and rough surface (Fig. 2B), all other specimens had a smooth appearance with entangled particles JOE — Volume 35, Number 9, September 2009

Figure 1. (A) XRD patterns and (B) Fourier transform infrared spectra of 5 calcium silicate powders sintered at 800  C. R, CaO; ;, b-Ca2SiO4.

(Fig. 2C F). Moreover, it seemed that S50C50 had a denser structure than the other cements.

Setting Time The setting times of the 5 CSC cements ranged from 12–42 minutes (Table 1); these values were significantly different (P < .05). Physicochemical Properties of CSCs

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Figure 2. (A) XRD patterns of 5 CSCs. SEMs of (B) set S70C30, (C) S60C40, (D) S50C50, (E) S40C60, and (F) S30C70 cements. R, CaO; ;, b-Ca2SiO4; >, C S H.

Compressive Strength Table 1 also shows the CS values of cement specimens. One-way analysis of variance of the CS data showed that the variations in strength between specimens are significant (P < .05). pH Variation There was no significant difference (P > .05) between the pH values of different CSCs at the same time periods. The pH value at fresh mixing was between 10.4 and 11.1, indicating a slight increase with an increased CaO amount. After 1 hour of setting, all cements reached a pH value of 12.0. By 6 hours, they approached a steady state of up to pH 13.6.

Discussion In the 1990s, Torabinejad et al (13, 14) developed a Portland-like cement known as MTA. After that, the formulation of hydraulic calcium 1290

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(alumino) silicate cements was investigated with respect to their potential clinical use in dental surgery applications (2, 3, 12, 15–18). In this study, the sol-gel technique was used to prepare 5 different calcium silicate powders. The XRD analyses indicated that S70C30, containing the highest amount of SiO2, had an amorphous phase without characteristic peaks. With a greater amount of CaO than SiO2, the diffraction peaks of b-Ca2SiO4 became stronger, even with a small total amount of CaO. Similarly, the CaO peak intensities increased with increasing CaO content of the powders. In the FTIR spectra, the broad band of 1300–950 cm–1 further split into 2 appreciable adsorption bands, indicating increased crystallinity with increasing CaO content, which is in agreement with the XRD results. A new band at 850 cm–1 emerged that was associated with the Si–O symmetric stretching mode, with one non-bridging silicon–oxygen bond (Si–O–NBO) (19). The band at 550–500 cm–1 originated from the vibration of the siloxane backbone (20). The presence of Ca2+ as

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Basic Research—Technology the network modifier in the silicates led to a disruption in the continuity of the glassy network as a result of breaking of some of the Si–O–Si bonds and resulted in the formation of Si–O–NBO (19). The bands between 1550 and 1380 cm–1 might have arisen from the vibrational mode of the CO3 group, which came from atmospheric carbonation. The XRD and FTIR results of the powders consistently indicated calcium amounts significantly affected the phase evolution. The present CSCs exhibited distinctly shortened setting times as compared with the setting time of MTA (>2 hours) (21). This fast set reduces the risk of dislodgement and contamination when cements are used as root-end filling material (22). Particle size, sintering temperature, liquid phase, and composition of powders, as well as the ratio of liquid to powder, played crucial roles in the setting time of the paste materials (23). When the powder and liquid phases were mixed in an appropriate ratio, they formed a paste that hardened by entanglement of the crystals precipitated in the paste at body or room temperature, as observed by SEM (Fig. 2B F). The entanglement structure, consisting of fine particle agglomerates, could be considered a hydration product of a C S H gel (Fig. 2A) that might be responsible for causing the particles to adhere to one another. In analogy to the setting reaction of calcium silicate–based Portland cement (24), it is speculated that the product was related to the hardening mechanism of the present CSCs. The setting time was significantly inversely proportional to Si/Ca ratio. This result might be interpreted by the amount (or crystallinity) of the b-Ca2SiO4 phase that could affect the formation of C S H gel. Higher C S H content in the final hydrated product will yield a better and faster hydration reaction with a shorter setting time (25), as observed in the present 5 cements. The peak at 2q = 37.5 was attributed to a CaCO3 phase, which decreased after mixing with water, and might have also contributed to the shortened setting time. The 5 CSCs were alkaline, possibly as a result of the release of calcium ions (24, 26), similar to MTA (7, 14, 16). The S30C70 cement had a slightly higher pH because of its high CaO content, which transformed into Ca(OH)2 when set. The cement specimen with the greatest SiO2 content (S70C30) had a strength of only 0.3 MPa, whereas the specimen with the lowest SiO2 content (S30C70) reached a strength of 3.2 MPa. It is worth noting that the highest CS value was 15.2 MPa and belonged to the equimolar ratio of SiO2/CaO (S50C50), which appeared denser in structure than the others. Although the detailed mechanism underlying the changes in CS has not been fully clarified, differences in surface structure, such as porosity of the hydration products, might play a crucial role in the mechanical properties of cement (27). The mechanical strength of the cements examined here decreased with increasing porosity. The Al-free hydraulic CSCs that exhibited shortened setting times were successfully developed. Among the 5 cements studied, both S40C60 and S30C70 might prove the most useful for endodontic treatment requiring a setting time of a few minutes, such as root-end filling/ sealing and pulp capping/cavity lining. Additional studies, including biocompatibility tests, are currently underway to evaluate the clinical potential of quick-set CSCs.

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